Which would you build and how is the reliability?

7.62 .......... Basically a .30-30 semi auto. Magazines continue to be problematic, though the NEW C-Products are very reliable with 26-28 rds. The days of truly cheap ammo are gone. One can find corrosive for eighteen to twenty cents a rd though. Cleaning after corrosive not that big a deal...but MUST be done at some point.  I've yet to see a 7.62 that was close to a 5.56 in the  ACCURACY dept.

9mm...weak rd, reasonably cheap to shoot, short range proposition. IMHO unless you have 5K or more of FREE nine...not worth the trouble.

5.45x39.... Cheapest ammo  currently available (7N6 surplus) .  Very similar to 5.56 balliistically. Reasonably accurate.  Simply use downloaded P-Mags with 24-26 rds. OR buy the C-Products. Mags can be an issue, but far easier to find SOMETHING that works with 24-25 rds than the 7.62x39 guns.

No warranty expressed or impliied, my opinions the result of  40 years of rifle lunacy.

FN in MT

You have a very good point about the 5.45x39.

Neither, I agree, A lot of problematic magazines. I had a Colt 9mm AR 6450 and I sold it after the newness wore off...I'm not a big fan of the other calibers either. JMHO.....

9mm are fun if you run them as SBR with a suppressor.  I have one and it is fast becoming one of my favorite guns.  With that said, I would pass on a 16" version as it does nothing a 5.56" doesn't.  I shoot at an outdoor rifle range, so I don't have to worry about range ratings.  But if you can only shoot pistol calibers, then I might go with a 16" 9mm set up.

I have a 10.5" 5.45x39mm upper inbound to me as well.  I am mainly getting it to try something new.  It won't do anything my 11.5" 5.56 upper won't except shoot cheap surplus.  But, my 11.5" shoots well enough with .223 Wolf which isn't that much more.  I really would like to try a 7.62x39mm or 6.8 SBR upper next as well.

I built a 7.62x39 last and love it. I killed a pig with it a few weeks back and am going to take it deer hunting this year. It was fun tweeking out the bad and making it good Its not a true "battle rifle" but It is fun to play with. I have other ar's that are go to's but this one has been great. I have a thousand rds of brass case ammo and a few hundred steel the brass is great and the steel is hit and miss. I also load for it. I dont have to clean it right after like I would with the more corrosive other ammo choice and its better then the 9mm. IMO if im gonna shoot a 9mm ill do it with my pistol.  I had one and after a few hundred rds I sold it. I think I liked the idea of it before I had one. Then after having one it really wasnt that big of a deal. Now im leaning towards the .300 whisper. I already went through the 6.8 stage and have .308 and 5.56.

Toss up between 762 and 545 for me.  545 for the cheap ammo and 762 just because it's a cool caliber that is not much more.

I just finished the 5.45x39 build. A fun cartridge to play with. Cheap to shoot,accurate at 600 yards,flat shooting.
Don't rule out the 6.5 Grendel or the 6.8's out there. The 6.5 G I built last month is a long range tack driver.

Is your Citadel build still running good?...I have yet to do the extractor mod on mine and get it out again.

none of the above simply because of magazine reliability .