Or, Death From Below. Frankly, it's a beautiful kit, though there are some sweet conversions out there (like this 2007 one I found on a French guy's site). That aside, the Trygon and Mawloc compete for slots, and have some different capabilities. I'm going to go over their similarities, their differences, and ofter some thoughts on their employment.
They're both deep-striking monstrous creatures with a pretty ridiculous amount of wounds. They're both pretty reasonable troop-killers, and as monstrous creatures vehicles ought to be wary.
The Deep Strike
The Mawloc's chief use requires deep-striking, and the Trygon still benefits from it. Neither of them can mishap on the deep strike unless they scatter of the table, though. In that regard, deep-striking them only subjects them to reserves rolls.
Note carefully that you cannot assault on the deep strike. However, it's hard to end up out of assault range with the natures of their deep-strikes.
Both of them drop S6, armor-ignoring wounds on things on the attack. As monstrous creatures, when they assault they also throw 2d6 for armor penetration. However, they only hit at S6, which means multi-wound T4 boggarts can be...annoying...for you, to say the least. At least you can get poison and furious charge to give you a little more lethality.
Six wounds, T6, 3+ armor saves. Ouch. That's a whole heap of wounds to deal with. There's a good chance you'll survive to make it into hand-to-hand, though you do worry about powerfists. Also, should anyone wander up to you with a bonesword and manage to land a wound...ouch. Ld8 base is potentially worrisome, since you are NOT immune to instant death. However, it does take specialist kit to inflict that instant death on you (IE: Force Weapons, Boneswords, small children and pets rampaging across the table...).
Feed. IE: go forth and try to eat that thing. Honestly, while the enemy CAN bait you with expendable stuff (or stuff you don't want to assault), you weren't really taking these for the shooting, were you? I mean, if you wanted shooting in the heavy support slot, you would've bought a Tyrannofex, right?
The Difference is in the Damage
Honestly, there's only one real difference between the Trygon and Mawloc: how they do the damage. The Trygon generally wants to carve you into little pieces, and the Mawloc wants to take a big ol' bite out of you.
The Trygon: Stabble-stabble-stabble
On the deep strike, the Trygon's essentially a drop pod: if he would scatter into another unit, reduce scatter so that he does not. (or she, if you prefer; I'm sure an Aliens-like head crest would look nice on a Prime). On arrival, you can choose between firing a half-dozen S5 shots (or a full dozen if you upgraded to the Trygon Prime) at BS3. Or, you can run.
The turn after that, you're a monstrous creature (so you have Move Through Cover) and you're fleet (so you get a run). That's a pretty dangerous combination for something that just plunked down pretty much right in front of your lines. Then, you have six base attacks at WS5 that re-roll misses in close combat, since you have two sets of scything talons.
You worry about hidden powerfists in units, and running up against units full of S6+ power weapons. Anything else is meat. With I4, there's a good chance of ruining I3 walkers before they even knock a wound off, and even Ironclads step lightly around you. If you can catch a Land Raider that moved slower, you might be able to do some damage (...but then, y'know, there's the CONTENTS of said Land raider to contend with...).
On the bright side? Once you reserve in, it's pretty much a case of 'run here, assault this.' You'll arrive, the enemy has a turn to run away or kill you, and then you should be in it. A faster army might give you some trouble (IE: fast skimmers that can afford to relocate).
The only other real note is that you can upgrade a Trygon to a Trygon Prime and get yourself a Ld10 Synapse Creature that popped up right in front of the enemy lines.
The Mawloc: Om-nom-nom
And here we get to the crux of the matter: while the Trygon is a relatively straightforward assault machine, the Mawloc's use is a little more nuanced. It's this not-so-little rule called 'Terror from the Deep'.
Basically, you WANT to deep strike onto the other guy. When you emerge (ideally under people) you lay a large blast template where the Mawloc would emerge, and everyone under it eats an S6, AP2 hit. Vehicles take the hit on the rear. Then, the Mawloc gets placed, and everyone gets shoved 1" away from the base. If you're in assault, you stay in assault unless you just CAN'T. If you can't be moved out of the way period, you're dead.
Some caveats on this:
-This is not a monstrous creature assault hit, so you're not getting +2d6 armor penetration
-There's a bit of a question on whether or not cover applies. It's not a shooting attack, though; it occurs in the movement phase (the closest rules analog would be the Eldar Prince Yriel's Eye of Wrath special attack)
-Resolving this is going to just get...interesting (and fun)
Once the Mawloc hits, shoves everyone around, and generally makes a nuisance out of itself, it's got 3 base attacks at WS3, and I4. The I4 only matters because of hit 'n' run. Additionally, the mawloc can put itself back into reserve, and automatically enter the next turn. However, it cannot put itself in reserve on the same turn it arrives.
The easy way to deal with this (as Faolain noted) is you start the Mawloc(s) on the board on turn one. They burrow, go back into reserves, and arrive on turn two. Then you can pull them back on turn three, launch them again on turn four, and turn five you get the choice to pull them back and gamble on turn six, or keep them where they are.
Damage and Reliability
Now, when you get down to it, you're paying in the 170-200pt range for either of these. The durability is the same, the reliance on reserves is about the same, and it boils down to who slings more damage.
Here's where I have to lean towards the Trygon. The Trygon arrives, gets shot at for a turn, and then probably gets into assault. The Mawloc arrives, hopefully lands on something (as you ARE rolling 2d6+scatter, so you hit on-target 33% of the time) and then considers going back into reserves or assaulting something. The Trygon cranks out a pretty high number of attacks with re-rolls to hit. The Mawloc might be able to eat a stationary vehicle, and has a 66% chance of hitting and running so it can go back into reserve.
Note that if you look carefully at the rules, at the moment there's nothing in the Mawloc rules that allow it to deep-strike onto a target! You MUST deep-strike no closer than an inch to the enemy, and hope like hell that scatter onto the enemy.
This kind of kills the Mawloc for me, until they FAQ it.
The problem lies in getting the Mawloc's ability off. Suppose, best-case scenario, you get a Mawloc in on turn two. It arrives, eats something. Turn three, back into reserves. Turn four, auto-arrive. Turn five rolls around, and you're left with a question: do you put yourself back into reserves and hope there's a turn six (...which is pretty reasonable, since there's turn six on a 3+ barring time considerations) or just go to town hitting and moving things? The problem is that if you're wrong, and the mawloc ends the game in reserves, the enemy gets credit for destroying it. Whoops.
The trygon...just gets there, and starts cutting things.
Why Look at the Mawloc, Then?
For a minute, let's go look at the Callidus assassin. She has the ability to move an enemy unit during their deployment. Do you know what else lets YOU move the enemy? There's the Lash of Submission, which can be fun with plasma cannons against people not in a transport. There's the Pavane of Slaanesh, which hardly ever sees use (...seeing as how you need the Masque, or a Herald of Slaanesh).
Now, what abilities do you know of that damage the enemy, AND let you move them?
What happens when he's got a scoring unit on an objetive, and you damage them, force a morale check, AND slap a six-wound monstrous creature on the objective?
Can you think of some nasty uses for this? Yes, you can. You might not even HAVE to go back into reserves. You might not even have to win the assaults. If the enemy can't kill you in CC, and you can wander around objectives with a Mawloc, you can cause some serious mayhem. Now, a Trygon will be nasty in CC, but you can potentially screen higher-value targets from a Trygon. There's nowhere to hide from the Mawloc. It can hit you, wherever you are on the board.
It's this ability to potentially hit anywhere that makes the Mawloc so nasty. It's kind of like ordnance: flinging large blasts makes people spread out. A mawloc makes people spread out, and makes them worry that it will pop up somewhere inconvenient. I think that a lot of people might overlook these tricks.
Things to Use With Either of Them
Well, first off, take a couple of them, either way. Redundancy is your friend, unless you're throwing a Trygon out as a wild card.
Second, consider reserves manipulation. Trygons can start on the table, but you're still moving 6" and running to make it to the enemy. Consider a Hive Tyrant with the +1 to reserves upgrade to get the big cutty snake thingy on the table faster. Note that you also want guns. While either of these can damage vehicles, you'd really rather hit troops and just EAT them in shorter order.
The Mawloc can be kept in reserve, but given its ability to go back into reserves and guarantee arrival next turn, I'm not sure I'd ever start it in reserves unless you're playing Dawn of War, then I'd walk on, and burrow.
Additionally, you don't have frag grenades. Be aware of that fact. It'll only really hurt you if massed powerfists/relic blades/Grey Knight Terminators have cover, but I had Black Templar termies in cover knock four wounds off a Trygon just because the Trygon didn't get its I4 on the assault.
Finally, a word on regeneration: it's pricy, but I think six wounds is enough to keep you around to make some regeneration rolls. One-rounding a carnifex isn't out of the question, but those two extra wounds make a difference. Unless you meet massed melta/plasma that's in a position to deal with you, you're gonna last. However, if you DO take regeneration, take it on pretty much everything that can take it. Otherwise, regeneration just means 'One-round me or ignore me in favor of something you can one-round.'
Both MC 'chassis' are hefty, resilient, and will never see a cover save. Neither of them are god-like in assault, but can do some serious damage to people without specialist kit.
The major difference is this: do you take the Trygon for its simplicity and brutality, or do you take the Mawloc and do dastardly deeds with Terror from the Deep?