first impressions, time for me to actually sit down and think through this. I'd recommend this approach to anyone - first impressions count, and so does the more thorough read-through.
For this run-through, we're going to assume that KDK is going to be my primary (if not only) detachment, and I'm going to build around a Warlord in there.
So, we've got a couple of choices here, really - do I want to run a Bloodhost, or do I want to run a CAD? Or, do I want to run both?
Detachment Perks - Efficiency in Blood Tithes vs. Obsec and freedom in builds
Option 01 - Bloodhost
Note that we're going to look at the Slaughtercult first - because it's your core choice for this. Then, we're going to look at the limitations of the Blood Host when it comes to building out from there, as you must take at least one auxiliary choice.
The Slaughtercult gives you four perks - Blood Frenzy, Boon of Khorne, Blood Sacrifice, and obligatory warlord trait re-rolls. The Blood Frenzy and Boon of Khorne are your moneymakers off the bat - you get a free blood tithe point in a turn, AND when you spend blood tithes you can buy twice if you have the points. So, you're generating more points (in addition to MURDER and offering your own skulls, that is) AND you're potentially more efficient.
Blood Sacrifice is nice if you brought cultists - when those guys flub a leadership check (...assuming they aren't murdered outright) you can just sacrifice them and get that Blood Tithe. You're making these guys as cheap as possible because they're either going to provide a screen, or you're going to try to snag objectives with them and don't want to devote points to that when you could be KILLING.
First off, you're committing to taking a unit of Possessed. If you want them and a Rhino, they're 185 on the dot for 5 guys and a ride. Possessed aren't bad, but I'm going to assume that you're going to buy them a rhino. If we go with a naked Herald and minimal Bloodletters (absolute minimum required in the slaughtercult) then you're looking at 430. Not bad, but a Herald is kinda 'meh' and seems like he's in there to keep it cheap. Plus, he's a character that can't explode into a Daemon Prince or Bloodletter through tithes, which is sad.
You CAN grab a lightweight Bloodthirster as your HQ, but that's a couple hundred points above your herald. You can, if you want, grab Cultists or Spawn, but Cultists are screens that turn into Tithes, and Spawn are bullet sponges either on their own or for a Lord taking a speedy choice.
Note that your core choices are gonna be power-armored bobs, deep-striking demons, and some choices for HQ that range from 'cheap herald' to 'lord with crap-ton of options' up to 'daemon prince' and 'Bloodthirster.'
Now, the Auxiliary choices are where we really start to see the price of taking a Slaughtercult. You're taking tradeoffs now.
I see two immediate perks - you can grab the Gorepack (swift disruption) and you can grab your nominal heavy support choices as an aux choice. These mean you're thinking about a faster army - the Gorepack involves scouting Khorne Dogs and bikers, and the bikers are either thinking about melee (and possibly escorting a lord) or they're small and packing meltaguns (I'm a Blood Tithe OR a messed-up vehicle, your opponent's pick). I feel like those would mesh well with your Maulerfiends (who also move 12" and care not for terrain) and anything you're bringing that has wings.
Now, there are drawbacks - if you want Raptors or Terminators, you're going to have to grab some extras - your Terminators only come out to play if you've got Bloodcrushers, which are. Um. Either jogging across the field with a laughable save, OR deep-striking assault troops with a laughable save and no shooting. Raptors aren't bad, but have to be taken en mass (read: 2 units of raptors and a unit of Warp Talons).
I'm not saying Talons/Raptors are bad, but I am pointing out the opportunity cost - if you want to bring 'em, you'll be looking at more than 400 just to field the minimal units - but at least the Helldrake isn't mandatory with Khorne's Bloodstorm.
My gut feel is that the Slaughtercult wants you to take a decent number of fast, assaulty units. Guns are what you use to make noise while running at the other guy and screaming. Or it's what Soul Grinders have and the rest of your army doesn't.
Option 02 - Combined Arms Detachment
You can always skip the Blood Host, and start out with a CAD and formations. You are losing out on the aforementioned perks of the Blood Host - which is the efficiency of the tithes, and the ability to take a crap-ton of heavy support choices.
Combined Arms Detachment Perks
So, first off, obviously there's Objective Secure. That's one thing. Considering you like to hit things, and your troops are punchy, well - if you don't absolutely crush something in assault, your troops can be all "Hey, we're Obsec and you're not! Also, please let us kill you in your assault phase so we don't get shot at."
Now, here's the other thing - you've got some flexibility in your build.
You're still stuck bringing an HQ and two troops. Now, if you want to keep it cheap, you can take cultists and/or Bloodletters. If you wanted to grab two Bloodthirsters (and wanted more than one of them to be the D Thirster), go for it.
Now, there are some actual options this opens up - you're not stuck taking units in gobs. If you want to grab some Terminators (...and probably Deep Strike them with combi-weapons) then you can, because you're not stuck grabbing a handful of Bloodcrushers as well. If you want a lone Raptor unit, go for it - but note that you can still take the Gorepack if you really, REALLY wanted to throw in a ton of Fast Attack options.
You could also grab a Land Raider without grabbing a Terminator squad - but seriously, it's still a Chaos Land Raider with long-range guns and an assault ramp, which still seems confused.
To me, seems like the biggest perk of taking a CAD is double-tapping on Bloodthirsters with D-strength axes, since you just can't do that with a Slaughtercult. That, and Terminators.
Also, as discussed in the Allies part, you can grab a Lord of War - the Kytan and the Chaos Knight forge world rules are Lords of War, and those need a CAD.
Honestly, I see a couple of limitations - if you really, really wanted to bring a lot of heavy support, well, you can't. AV12 and AV13 walkers really kind of like numbers. The other is that, obviously, you lose the fun perks of the Slaughtercult with respect to Blood Tithes.
Nothing says you can't take a CAD and throw formations on top though.
A Thought on Allies
So, there's one thing your allies won't have - the ability to produce Blood Tithes. You want your guys to either kill things or die trying - you're playing KDK for the fluff and for the Blood Tithe mechanics. This is the first thing you want to keep in mind.
The second thing you want to keep in mind - GUNS. You just don't have a lot of them. You're an assault army. There's kind of a second thing to keep in mind, and that's AP2 - you don't have a ton of that outside of Bloodthirsters or a Lord hitting things with an Axe of Khorne.
Lords of War
Honestly, there's a couple of things that aren't in the book that are worth looking at - the Kytan Daemon Engine and the Renegade Knight. The Knight can either come in Renegade flavor (1-3 per detachment, lots of options). If you want to go Forgeworld Experimental rules, you get the Kytan or a different flavor of Knight.
So, here's the comparison you need to do - Renegade Knight versus CAD with Kytan or Chaos Knight.
The Renegade Knight (out of Traitor's hate) will NOT have Blood for the Blood God. However, it will be cheaper than the Kytan, and potentially more shooty than the Chaos Knight. You have more options than the Forge World rules - which means more guns. The Forge World option is also slightly more durable since it gets the Daemon of Khorne rule and gets a nice 5++ in melee (and when the Ion Shield isn't helping it). But you're stuck with a CAD.
The Kytan is a little tougher (better side armor and worse rear armor; you have no Ion Shield but you get Daemon of Khorne. You get an 8-shot Krak missile launcher that somehow has pinning as well, and if you murder your shooting target, you can charge something else.
Honestly, the Chaos Knight is kind of in between the Renegade Knight and the Kytan. The Renegade can take two guns, and the Kytan is a bit more killy. The Kytan is about 100pts more than the Chaos Knight, but AV13 on the sides is pretty nice, along with It Will Not Die.
I'd recommend picking one or the other - grab the shooty of a Renegade Knight (along with the part where you don't have to go Forge World) or go big with a Kytan and enjoy the Forge World experimental rules and go axe to mouth on something. Plus it looks better than, y'know, the Lord of Skulls.
Seriously, you're taking other friends for shooting, including potentially aircraft, as your anti-air is the Soul Grinder and maybe Helldrakes. (...because that flamer is gonna do stuff, really.) Look for guns somewhere - if you're willing to either grab the formation OR pay the HQ/troops for it. and remember that any non-KDK guys ARE cutting into your blood tithes.
Why not CAD and Slaughtercult?
There are two reasons to take two - you're willing to take four troops, and you want the flexibility in HQs - which, really, honestly, the only 'flexibility' you get is taking a Bloodthirster with D-strength and you get the Lord of War slot (...which you'd fill with a Kytan, realistically)
If you're going to tack a CAD onto a Slaughtercult, you're doing it to get the freedom to take troops that are otherwise taxed - like Raptors or Terminators, and you want a Lord of War that isn't from an Allied Detachment. Otherwise, pick one or the other. You're looking at freedom in your build (and freedom from Possessed and other taxes) OR you get the fluffy fun of Blood Tithes for free.