Here's the scoop.
I have an 11.5'' and it runs well. That said, if you are going to run suppressed and want the shortest gun possible you're going to want to go with a 10.5''.
Forget 300 black out unless you are a reloaded or have a lot of resources or if you hunt or just primarily enjoy shooting a very expensive caliber bullet. Production hasn't picked up to make them anywhere near what 556 is for the money.
I have an AAC can. It works alright. I like Surefire products in general and think that they are high if not higher quality than AAC. You pay more for them though so you're going to have to figure out what you want to pay and if it is 'worth the extra' to you.
Don't get caught up in DB ratings and how quiet a can is. You should figure out what factors mean the most to you: Durability, size, weight, level of sound. If it is a full size can or medium sized can it's going to be quiet enough for all intensive purposes. If you want movie quiet then get a 22lr can and host.
Knowing what I know after shooting an SDN6 and having an M4-2000 on order...
You shoot an SBR because you want something short and light, then you throw a 17 or 20 oz can plus a 3-4oz muzzle device mount on the end of the rifle. It changes the dynamic of the SBR as a whole. It more so becomes a heavily pressurized regular sized (16'') rifle that is snappy and gassy with a 5-6'' heat container on the end. It heats up and stays super hot within 10-15 rounds. It does quiet the rifle and in most instances would make it more difficult to tell where the point of origin of the shooter is. It doesn't make the rifle hearing safe. I'm still wearing ear pro. It does make it much more pleasant to shoot primarily if you're running a muzzle brake on your SBR. With a flash hider on the SBR the suppressing only really takes the noise level down since the concussion is heading straight out instead of to the sides of the barrel / rifle.
What would I order now if I could go back in time?
If I'm sticking with 556 (which I am personally)... I'd get one hard use quick detach can that'd probably be a surefire can..it'd be 17-20oz + a mount, decent db reduction, great durability. I'd get another lightweight titanium thread on can that was 10-13oz. If I had multiple rifles that were high volume rifles I could use the quick detach can on those and then I'd save the SBR for the lightweight titanium one since I don't shoot at night (Ti cans spark sometimes) and I don't go crazy with full auto or mag dumps. Basically you'd save your hard use can for high volume shooting, and then you could use your titanium lightweight can for wanting a very light setup for hunting and or precision shooting. With how hot the cans get under sustained fire or high volume fire, along with the mirage that they will produce that can ruin your sight picture... I don't really see any real advantage to shooting them like this or practicality for it. If I'm hobby shooting and no one is shooting back at me, I'd rather have no can on it for high volume practice so the only thing heating up that I'm worrying about is the barrel and handguards (which also cool down much quicker than a can).