I agree with 7.62x39, especially as you asked which we think looks best. Some of the Hesse Saiga conversion .308's were sold with '74 style brakes (if I'm not mistaken). You might want to check some of the Saiga forums on the various sites for reference to see what others have to say, you'll certainly run into folks who've installed that style themselves.
I've never used an AK74 style brake on anything more powerful than x39 so I really can't comment on effectiveness. Personally with a .308 I would be more interested in performance than looks, especially with a semi-auto. Good muzzle control and tighter groups would be of utmost importance, especially when limited to the 8 round mags. I'd be interested in proven designs for the caliber. (Now if we were talking a Krebs hi-cap conversion I'd probably go for the 74 style for the "cool factor", not to mention they make one of the nicer US 74 type brakes)
Couple of links that might help point you in the right direction: