I have one of each
Heres my take...
300blk is AWESOME in a SBR package, but its a reloaders deal. Factory ammo is running just under $1 per round for plinking fodder and up to 2x that for SD ammo
556/223 is not as pleasant to shoot out of a short barrel. Ive had 7.5", 10.5", 11.5" and a 14.5" guns. They get louder and more muzzle blast the shorter you go. There is also a corresponding loss of velocity as the barrel gets shorter. I would NEVER recommend one under 10.5" in this caliber.
9mm does not have the power any of the rifle rounds does, but for a range plinker that doesnt matter. Cheap and readily available ammo, make this the best "shooter" IMHO.
Full disclosure... I run my suppressed 9" 300blk WAY more then my 556 or 9mm guns, but i reload for it and accecpt the additional cost of the caliber. As a HD/SD/Civil unrest gun, its my pick of the litter. Better terminal ballistics out of the short tubes then any of the others. The ability to just swap mags and run subsonic ammo SUPER quietly and still hit hard wins me over.
A 556 gun would be my 2nd choice. Even suppressed it going to be loudest of the bunch and with the really short tubes the round suffers ballistically.
9mm with 147gn is nice and quiet thru my cans, but its a pretty limited ctg at any distance at all.
Hope i confused the issue enough