Here's the scoop as I see it.
FAL- designed my M. Dieudonne Saive at FN in Belgium. It's a gas-operated tilting bolt design that is very durable and pleasant to shoot. The mags are wonderfully inexpensive, and a new DS Arms FAL is just as nice (nicer?) than an original FN. It's a hell of a nice rifle, if you get a good one. That means no Century parts guns. Get an original, or a DS Arms.
HK91 - I have one. Roller-delayed recoil operated weapon, no gas system and associated headaches. The chamber is longitudinally fluted to ease extraction (otherwise cases can be ripped in half - this system has no primary extraction phase whatsoever. If this seems counter-intuitive, please let me know and I'll go into more detail) This has the effect of marking up the brass with unsightly scorch marks. It still works for reloading, if you can find it, the 91 typically throws its brass 30-40 feet from the port!
It's extremely accurate due to its free-floated barrel. Yes, it is free-floated in its standard configuration. Mags and parts are ungodly expensive. Thermold mags work well when you can find them.
It does tend to kick the shit out of you in its factory configuration. This can be eased by changing the factory buffer for an MSG-90 buffer and adding the factory rubber buttpad. It's never going to be as pleasant to shoot as the FAL, M14 or AR-10 because of that huge-ass 2 pound bolt carrier beating into the rear of the receiver, but with the above mods it's livable. The clone guns are especially bad in this respect. No one is making a decent clone 91 these days. Avoid Special Weapons like the plague, Centurys ain't much better.
Bottom line- I love my 91, and I'd put it up against any FAL for accuracy and reliability but considering costs and such if I were going to buy a new .308 today, I'd get the DS FAL, without a doubt. The price difference between it and a preban 91 would buy you a shitload of mags and a bunch of ammo, with enough for a Thunder Ranch course left over.
YMMV.
QS