I have two ARs: a Bushmaster XM-15E2 and a Adcor B.E.A.R. Both are 16" carbines.

Planning on doing my first build in a month or so: AR pistol in 300 BLK.
But, in true ARfcom fashion, I've been buying in twos. Leaning toward building a 5.56 pistol using a 10.5" Rosco Mfg barrel.

I'm in no rush to do the second build so I have time to think it over... just getting caught up in the sales.

I have a few 5.56 ARs. A long 24", 16" and 7.5" pistol. I built an 18" 308 AR and now in the same boat as you are.

I'm leaning toward a 10.5" 300BLK. I reload for 308 so the case conversion/prep shouldn't be an issue. If you do not reload then 300 BLK may not be as practical for you. Its been a while since I shopped around for 300 BLK ammo so maybe it's more readily available these days.

AR pistols are a LOT of fun! Most people that handle my ARs favor the shorty just because its so fun to hold and shoot. 300 BLK would be whole lot more fun for the power factor.

If you don't already have a shorty 5.56 then that wouldn't be a bad idea at all. Same mags, same ammo, etc...

9mm?

the cases for a 300 blackout are made out of 5.56 brass . so if you already have a 5.56 then you have a source of brass for your blackout . I do share some of my 308 bullets with my blackout  . having owend an 11.5 inch ar and presently owning an 8.5 inch 300 blackout pistol I can say the blackout will be much more pleasent to shoot .

The first build is without a doubt going to be 300 BLK. The only thing I have to buy is the lower.
The plan is to SBR it.
The plan is to buy two receivers together... and I may just get a jump on the inevitable and SBR the second receiver before I ever do the second build.

Probably do the shorty 5.56.

Not a huge fan of 9mm ARs. For some reason it just seems like overkill for that round. I have a Cx4 Storm so I got the 9mm carbine thing covered for now.

Had one stripped lower laying around for years, but after getting a second one during a sale, built up two 10.5” pistols at more or less the same time.  One in 5.56 and the other in .300 BO.  Just do it.