If you're going to be using supersonic rounds, then it doesn't make a ton of sense to choose 300 blackout over 556. That's not to say 300 blackout isn't going to ruin someone's day, but the ballistics of 556 are more damaging in pretty much every configuration over any 300 black. Especially if you don't care about overpenetration, then then 556 is going to put the most damaging hole in the bad guy.
300 blackout is quiet, scary quiet, and not having to worry about deafening yourself while defending your home is one of the things I like about it. But 556 suppressed is still quiet enough that you probably aren't going to ruin your flying career defending your home, and you get all the benefits of the sheer ferocity of 556.
If you absolutely need the shortest SBR, or want a hearing safe home defense gun, then 300 black makes sense. That's what I have. But if you're going to use super sonic ammo I'd probably stick with 556.