If you can, try them both. Figger which one you like better.
For myself, having owned them both, the G3 is hands down the better rifle. Better sights, more accurate, easier to clean, easier to put a scope on. I can't say that reliability is any different, but the FAL adjustable gas system "feature" is really a liability. I hated that on my FAL.
You can get a nice low scope mount for the FAL, where as the HK rifles are borderline difficult to scope with their "claw" mounts... says DScottW
I will disagree respectfully here. If you clean your rifle, if you clean the chamber area, you will have to remove your scope mount from the FAL. When you put it back on, you have to resnug a dozen little screws, and your zero will have shifted. So you have to shoot it to zero it. But if you shoot it, you have to clean it. So you have to remove the scope base again, and lose zero. Because of this, the FAL scope mounting arrangement is not as good as the one for the G3. The claw mount on the G3 fits right on, and retains its zero. You don't have to remove it to clean your rifle, but if you do, it goes right back on to the same zero as the last time. +1 Gew.3.
Heat control is better on the G3 also. +1 G3
G3 has a floated barrel... generally regarded as more accurate. +1 G3
Sights are better on the G3 to shoot through, but I'd have to give the nod to the FAL for ease of adjustment. +1/-1
One thing you will want to do with the PTR91 is to get a paddle mag release installed. It makes the rifle so much more user friendly. -1 for not having it in the first place.
As far as reloading goes, if you use military brass, which is thicker gage, and a
port buffer, you can reload your empties just fine, assuming you can find your spent brass. (The G3 does have a strong ejection.) The FAL is supposedly adjustable, but on its lowest setting it seems to kick the brass as far as does the G3 with the port buffer installed. This is because the FAL kicks the brass in a high arc, whereas the G3 with port buffer kicks it downwards, but after it hits the ground the empty will tend to bounce a few times before coming to rest. +1/-1
The grip angle on the FAL is odd. The grip angle on the G3 is pretty good. +1 G3
The collapsible stock on the G3 is OK, it is "cool" but the standard stock is better to shoot. The para stock on the DSA FAL I shot was a terrible design. They put the rear sling swivel on the left where the thing folds. When you shoot the rifle, the sling swivel punches you in the upper lip! -1 Para FAL!
Modularity: The G3 wins here. To change stocks, it is the simple matter of removing the two retaining pins and changing stocks. Takes 15 seconds or so. The lower grip frames are readily interchangeable on the same basis. To change stocks, you have to change the entire lower receiver on the FAL. To change the foregrips is also much easier on the G3. +1 G3
Triggers: Both sport triggers with a stronger pull then necessary, but can be reworked to improve pull. I will tell you that the Belgian G1 I owned had a very narrow trigger shoe, and combined with the heavy pull, was not as pleasant to shoot as the G3. The G3 has a much better, wider, smoother, trigger shoe. Just better to shoot. +1 G3
Bipods: The FAL bipod mounts on the barrel, which when deployed, will tend to throw your shots, and cause the rifle to shoot to a different point of aim. Both heavy and light bipods on the G3 mount in such a way that the barrel is unaffected. Definite +1 G3
Safety/selectors: Both are designed to go off safe readily. The throw on the G3 is shorter to get to either semi or auto setting. Getting either on safe requires one to release his grip and reset the safety. I don't care for this "feature" either. +1 G3 for shorter selector throw.
The FAL has a last round bolt hold open. +1 FAL
G3 magazines have a robust feed box much less susceptible to damage than the feed lips of the FAL magazine. Both are rock in designs. +1 G3
Both are great rifles. I would be happy with either. The G3 fits me better than the FAL, and is easier to maintain, clean, and care for. It is stone cold reliable without the need for an adjustable gas system, which I do not see as a feature, but a liability. I hate cleaning that gas system.
Get which one fits you well and you like the feel of best. Both are good rifles.
As an aside: I understand these days that DSA is the top of the line for the FAL. One thing I noticed when shooting the the Para FAL by DSA is that it rings like a bell. Owner said it had something to do with the alloy and hardening of the receiver. That ringing was very annoying, and penetrated my ear muffs. Whenever you shot the thing, or charged a round, it just rang and resonated for a very long time. If I were to buy one of those rifles, I would insist on dropping the bolt from the BHO and see if it rings. Also, I've read on THR several examples of the DSA which would not feed reliably out of the box, and owners had to modify some area of the receiver over the magazine box. This is unacceptable on a rifle of that price... but that seems to be the standard.