Back to Space Marines for me...for a bit, anyway. I'm playing with list-building for Deathwing, but I feel this is relevant for marines in general.
Offhand, Dreadnought vs. Landspeeder seems like a bizarre comparison; one is an AV12 walker and the other is an AV10 fast skimmer.
The thing is? They can fulfill similar rolls, and it's worth comparing them. I'm going to take into account the 'gimped' options of the Angel codices, and the regular ones available to Space Marines. Chaos marines? Sorry, doesn't apply to you. Additionally, I'm not touching the Land Speeder Storm, as it's a different beast altogether.
Common Ground to Both Platforms
The Speeder and Dreadnought are both vehicles. As such, they are keenly vulnerable to AP1 fire, and melta weapons. They are both reasonably adept at moving and shooting; the Dreadnought gets 6" and everything whereas the Speeder gets 6" and everything, or 12" and one main/all defensive. Additionally, both are BS4.
Basic Differences between platforms
-AV12 all around
-Capable of assaulting vehicles (assuming Dreadnought CCW equipped)
-May assault; may massacre troops ill-equipped to deal w/ AV12 in assault
-May always fire all weapons when moving
-May simply walk through terrain; slowed as per infantry rules
-Speed; is Fast so may move 6" and fire everything, or 12" and one big gun, or 18-24 and get a 4+ cover save.
-May Deep Strike at no additional cost (...unless an Angel codex model. Sorry.)
-Mobility; may move over cover/terrain/stuff
-May go Flat Out to get a cover save
-May be tarpitted in assault
-More costly of the two options
-Must pay for Drop Pod if it wants to Deep Strike
-Must WALK anywhere it wants to go
-Durability; AV10 vulnerable to small arms and lighter, higher-rate-of-fire weapons
-May be subject to Squadron rules
-Terrain: moving into terrain means dangerous terrain tests and potential immobilization; it's a 1 in 6 chance but it means losing your mobility advantage.
Short-Range Anti-Tank (or Infantry)
Landspeeder Loadout: Multi-melta, Heavy Flamer
-70pts Marine Codex, 75 Angels
Dreadnought Loadout: Mutli-melta, heavy flamer, Drop Pod
-115 for Dread/35 for pod; Angels pay 130 for the dread/55 for the pod
I'm factoring in the drop pod to make them both fulfill the same role: sit in reserves and deep strike. The Angel codices are stuck with potentially sitting in reserve and moving flat-out; it's a bit of a sub-par use of them. Basically, the angel codices can't guarantee a first strike with the things.
The bottom line is that both of these platforms can pack a nasty short-ranged weapons compliment, and the means to get there. Speeders are swift and may deep-strike. The Dreadnought can ride its drop pod down, then grab a shot at short range.
Both are also potentially hamstrung by the reserve dice. If the enemy is short on light anti-tank at range, then the speeder may escape harm for a bit, and may be able to get around that. Also, you can simply not board the drop pod. If it's a 'vanilla' drop pod, though, you can guarantee a first-turn drop. Of course, your opponent can also react, since it's predictable.
Pros: Cheaper. Faster. If it doesn't get nailed on the landing, it can potentially get away and do some fast contesting tricks. Nigh impossible to melee away; as it lands at Cruising Speed (6+ to hit...).
Or, it may deploy on the ground, behind blocking terrain. It can then move 24" on the first turn, and the second turn is another 12" move, looking for the the 12" melta shot, or slightly closer flamer round.
Cons: Durability (more accurately, lack thereof). Chances are you're going into the teeth of the enemy, and they will have range for everything. Deep-striking into flamer range is iffy, but a bit more viable for the multi-melta.
Pros: More durable, and more threatening in assault. If the enemy can't rapidly relocate, you get to punch them out. Locator Beacons let you vector in additional reinforcements. If it's a regular marine drop pod, you can get an alpha strike off. Drop Pod ALSO prevents mis-haps. A lighter pro is that the dreadnought may always fire the melta and the flamer. It's a minor thing against lighter armor, but if there are infantry between you and the target? Free hits. Additionally, if it's heavier/better armed infantry, the multi-melta can help out.
Oh. And, TRY killing one in assault. You will require specialist kit to do it, and a drop pod makes an assault dreadnought a very real threat.
Cons: Cost. You had to pay for the drop pod. You are also close enough to be tarpitted, and you are delivering yourself into melta range.
Stupid Drop Pod Tricks: you should pretty much ALWAYS be able to land in 12" melta range. The Drop Pod may also be used to block line of sight. The drop pod can ALSO sit on an objective. You may not be claiming it, but he'll have to kill it. In fairness, open-topped AV12 is not exactly are what we call durable, but you still have to devote actual anti-tank weapons.
The Speeder will always win on cost. However, the vanilla Marine dreadnought has a potential advantage with its drop pod; it can predictably arrive on turn one. On the flip side, it means the opponent can react to it. The vanilla drop pod also opens up a Locator Beacon option, which lets it act as a beach head or forces the opponent to pop another AV12 vehicle.
Personally, I have to swing a bit for the Landspeeder, here. For the price of a single dreadnought with pod, we can get two speeders. Thing is, any way you cut it the units are sacrificial. You're handing the enemy an armored threat in melta range. On the other hand, a Dreadnought does require dedicated attention. If you manage to drop 2-3 dreadnoughts into the enemy's lines...it's certainly a sucker punch.
Long-Range Anti-Tank (and/or anti-infantry)
Dreadnought: 2x Twin Autocannons (125); or Twin Autocannon/Missile Launcher (135) for Angels.
(Note that you can also go TL Lascannon/ML, or TL Las/TL Autocannon...)
Speeder: Heavy Bolter/Typhoon Missile Launcher (90)
-N/A on anti-tank for the Angels; their Typhoon is inferior
-The Angel codices simply cannot engage tanks at range
Common Ground (Such as it is)
Hey, neither of them has to get close. Dreadnoughts can sit back at range and fire; the speeder can use its mobility to dance around. This means we're generally dealing with purpose-built anti-tank weapons.
Pros: Higher volume of fire. Twin-linked autocannons wound infantry reliable and hit light/medium armor up pretty harshly. Missiles aren't bad, but the lascanon is probably a bit pricy for what it gets you. Two ranged anti-tank weapon systems means redundancy (which is good).
Cons: Slower. Potentially easier to get out of LOS. If you manage to get stuck in assault, you'll need help getting bailed out. Couple WS4, S6 attacks won't cut it.
Pros: Speed/mobility make it harder to hide from
Cons: Less firepower; either two S8 shots or three S5 and two S4 small blast shots. Single anti-tank weapon system.
I have to side a bit with the Dreadnoughts. Get them a decent vantage point, and they'll hold up to more anti-tank fire. Here's a point where I think the Dreadnought's durability starts telling: It ignores S5-6 weapons, and isn't terribly worried about S7. On the flip side, all these weapons come in a high-enough volume of fire to start to really trouble the speeder. The dreadnought also has a backup weapon system if it eats a 'weapon destroyed' result. The Speeder? I think the Typhoon is a fine weapon system, but it's still just a single weapon system, and it goes bye-bye with a single damage result.
The Conclusion Bit
This is admittedly a bit of apples-to-oranges comparison. You have two different ways of fulfilling similar roles. Admittedly, the Dreadnought is more versatile if it keeps the Dreadnought CCW, as S10 attacks that ignore armor saves may be hazardous to your health. It also pretty much auto-pens vehicles. Of course, you have to get close to the enemy and actually risk melta weaponry. You can bet that if the enemy feels like you might assault them, they're going to try to take out your CCW at the least.
The Speeder, though, is a nimble opponent. It can run behind terrain, and can dart forward to readily bring its short-ranged, deadly payload to bear. Problem is, the speeder is not exactly a durable platform. Between its lethality and lack of durability? Expect it to get shot up. Then again, it's also cheap.
The Speeder wins out for the more dangerous missions, as it's a bit more expendable. Plus, there's something satisfying about trading a 70pt speeder for a 100-200pt tank. Doesn't always happen, but might as well go for the gusto, yes? Yes.
The drop-pod Dreadnought is a nastier threat at close range because of its armor, melee threat, and ability to arrive a bit more reliably with the deep strike. Ultimately, though, I think that if you're adding enough Drop Pod dreads to be reliably, you're pretty much dedicating to a full or at least partial drop-pod build.
At range, though, the Dreadnought beats the speeder out hands-down. The paired twin-linked autocanons is simply a superior combination of firepower and durability. 90 points for two krak missiles on a fast platform, or 125 for a pair of twin autocannons on a more durable platform? Again, the speeder's inherent fragility will work against it. "Hey, I can pretty readily kill a double krak missile launcher? SIGN ME UP." Dreadnought, on the other hand, takes a little more abuse and heavier fire.
Note that I'm cutting out the twin-linked lascannon. Again, honestly, a lascannon won't do that much against heavier armor. A twin-las has about a 5% chance of popping an AV14 target, and 9.7% chance of killing an AV13 target. Versus AV12, the lascannon will score 0.146 kills, and the twin-linked autocannon will score 0.097 kills. Not a huge difference, but a 0.5ish swing fowards the las. Versus 11, lascannon scores 0.195 kills, and the autocannon gets 0.293 kills. Pretty clear winner here. AV10 sees the lascannon score 0.24 kills, and the autocannon gets 0.293 kills.
I suppose you could always go weird and grab a TL las and TL autocannon, but to be honest, any time you really want to engage the enemy's heaviest armor you're forfeiting the autocannon's utility. Just get the capability elsewhere in your army.
What's the best part of this, though? Dreadnoughts and landspeeders do not compete for force org slots. Dreads are elites; speeders are fast attack. There's simply no way for them to compete against each other. As such, it's perfectly reasonable to, in a large marine army, take three twin-autocannon Dreads for 375 and a minimum of three speeders for 210. That's about 1/3 of the army. Not bad for the capabilities you get, and at the end of the day? Marines are just reliant on melta to kill larger tanks. It's a fact of power-armored life.
What about the Dreadnought's assault cannon as a heavy anti-armor weapon? Relevant on the drop pod version of the dread, or a speeder.
Regardless of range, Assault Cannon gets: 2.66 hits; 0.44 rends on average.
vs AV10: 0.88 pens, 0.295 kills
vs AV11: 0.44 pens, 0.14 kills
vs AV12: 0.44 pens, 0.14 kills
vs AV13: 0.293 pens; 0.097 kills
vs AV14: 0.146 pens; 0.048 kills
Multi-melta out of melta range: 0.66 hits, but kills 50% of time rather than 33%. Plus, kills 1/6 time on glances, so an extra + 0.036 times
vs AV10: 0.44 pens, 0.22 kills > 0.256 kills
vs AV11: 0.33 pens; 0.165 kills > 20.01 kills
vs AV12: 0.22 pens, 0.11 kills > 0.146 kills
vs AV13: 0.11 pens, 0.055 kills > 0.091 kills
vs AV14: kills 0.036 times on the glance.
Bottom line? Against middle-to-heavier armor, the multi-melta can still hang with the assault cannon in terms of kills. It doesn't have weight of fire, but the AP1 helps it out. Oh, and once you get in melta? The assault cannon actually does about equal against AV13, but that's really it.
vs AV10: ~33% chance of kill
vs AV11: 31% chance of kill
vs AV12: ~28% of kill
vs AV13: 24% chance of kill
vs AV14: 20% chance of kill.
Some rounding applies, but...yeah. We all know melta weapons are sweet, but rend-o-matic just can't compete with the melter.