The HK91, FAL and M1A are all excellent rifles. You can find licensed, authorized and unauthorized copies of all three, in grades ranging from good to bad to ugly. On the origionals, all have very good sights, are very reliable, and are accurate enough. Down side is they can all be pricey, the HK roller lock as a rep for beating up brass and the cocking handle is in a funny spot if you are lefty, the FAL is sometimes a little finicky about ammo and cleanliness, and the M1A's safety is a little odd and it is probably the heaviest. As was suggested, make sure it is chambered for .308 Winchester rather than 7.62NATO- the former is hotter.

So, if you've settled on a "tactical" rifle, great. But I also like to toss out things like the Remington 7400 to think about. Politically neutral, just as accurate, sights are very good, available in true .308, and not being a "tactical" rifle it is less expensive. Downsides are that the biggest magazines I've ever seen for it are 10 rounders, and it won't take as much abuse as a rifle originally designed for military use.