Well, if it were me, and I already had a short .308, I might go 28" but in a different caliber. Nothing against the .308, but why not try something different? One of the main reasons I have a .308 (it was the last of about 8 different cartridges/calibers I ever bought) is I wanted a good shooter with a short barrel. If going long, why not try something that really shoots better with a longer barrel? How about some kind of 6.5mm??

Edit:
oh, maybe 'cuz you said you want it for F-T/R