7-08, though as Casey said, it's kind of a lateral move. They are... shall we say... pretty comparable.
Are you doing this for recoil? If so, and since you are building this rifle (it appears) you might consider a fast-twist .233AI. While it's no .308 in the deer woods (BSEG), it a cool target round. I have a medium-twist heavy .223 that I am shooting the snot out of; when it's toast, it becomes a fast twist .223AI. The fast twist let's you run heavy bullets, and the AI adds just enough poop to move em fast enough to be useful, and the .223 means cheap brass. And no recoil.
Anyway I'd hunt the .308 and build the target rifle, 'twer me...