Of course I'd choose the HK91 as my personal favorite. I'm used to the HK weapons system. I like the ease of swapping out parts, in particular putting a PSG1 trigger group on it takes seconds. You can put a collapsible stock on it if you want to abuse your shoulder. Easy to swap out forearms, bipods, adj. stocks (cheek piece and length), etc... I also like the irons on the hk91s (I use a 1200m sight on mine). I've never had a hiccup and I think it's as accurate as I need out of a battle rifle in semi. Best of all, if you got a registered MG sear, you can drop it in and have a full auto G3!
I don't mind the lack of a bolt hold open. The negatives are: it's a bitch to clean, parts and mags are expensive, and it gets VERY dirty quickly.
I want to pick up an AR10 (better yet an SR25) one of these days. That'll be my next .308.
I really don't think you can go wrong with any of the following rifles: HK91, M1A, AR10 or a FAL.