This is another ongoing (9mm vs. .45 type) arguement.
My views--I owned an HK 91, several FAL's (DSA Standard, Carbine and SAR-48), an M1A (sold it unfired, but have shot another one) and some AK-47's.
Skipping the AK vs. AR arguement (since the 91 is a .308 battle rifle vs. a .223 or 7.62 x 39mm assault rifle), I see it this way (ranked in order):
Accuracy:
M1A, HK, FAL.
Durability:
FAL, HK, M1A.
Ergonomics:
FAL, HK, M1A (personal preference plays a large part in this one).
Reliability:
HK, FAL, M1A
Recoil:
FAL, M1A, HK.
I may be wrong on one or two orders, but it should be close to that.
I personally prefer the FAL--mags are cheaper, parts are cheaper, and I like the way it feels. The HK has better sights and is easier to scope (the FAL dust cover, even at it's best, can slip). The M1A is more conducive to accuracy and accurization.
It is definitely a personal preference thing.