Shot both, owned both and sold the pre-ban HK91.
The parts and magazines for HK91 used to be expensive. Thank god, new 20rds alloy mag can now be had for about $7 each. Parts kits also easier to come by. I didn't really like the recoil- beating me to death, also took longer for me getting back on target when shooting off hands. I also hated the magazine release and the cocking handle which was way too far forward. The other thing I disliked was collapsible stock- never felt comfortable w/ it. However, I still think it looks pretty nice, as I still have a post ban one w/ FMP receiver but I don't shoot it anymore.
On the FAL, I picked up a Pre ban one three years ago- still have it. It now has a shorter barrel on and a para folder which is more comfortable than HK. The controls are much easier to manipulate along w/ the adjustable gas system for different type of ammo. The rear sight on most FAL is horrible, but some company (TNT, I think) is making ones just like those on AR-15A2. I never did like the FAL that much, but after reading quite a bit about it, I later changed my mind.
I now own another FAL: a DSA STG-58A, along w/ a few sets of parts kits. The STG-58A is very well made, very affordable for 308, and accurate as well, as another members mentioned before.