If mag are your deciding factor-the 308. You have more available and can convert to fit. The 12 on the other hand are difficult to find and expensive. Three eight round mags will cost you more than the gun. That being said- there's nothing like the reaction you get when you walk on the range with an S12 in hand. Even the AR stodgie (you know the guy) will grudgingly want to check it out.
I would get the S12. Really, come on now, imagine that with a bayo lug and the barrel cut to 16" but made legal by a perm. flash hider. That would look AWESOME, and add the folding stock... man, crazy. Get a 10 round mag and bump fire it at the range. :twisted:
OR, invest $200 and legally register it as an SBS... a S-12 with a 15.5" barrel and all... even crazier.
Let, these guys decide http://tromix.com/
Thats a hard call for me, Though......I dont know what I would do. Probably the 12 with a 12 inch barrell or so first.....then later the .308.
If you are wanting something for fun and just shooting stuff get the 12. If your looking for something that you are gonna shoot targets with and maybe hunt with go with the .308. For home defense use the 12. If its gonna be for bugging out get the .308......So many factors makes me want both of them, so little money makes me get neither.