I had a 91 I bought just before the big AWB in the 90s. I got it for just over $600 and sold it for over $2k in the late 90s. It was a well made rifle but I have to say I don't miss it. I much prefer my DSA FAL over the H&K (and I'm a HUGE H&K fan) and I like my Armalite AR10 even more. I know this is a topic that has been bandied about greatly, but I'll be glad to give my $0.02 worth.
H&K 91
Pros: Reliable, high quality build, simplicity of takedown, and the best open sights I've ever used on a rifle. I love the rotary sights which allow you to select 100, 200, 300, or 400 meters. My wife and I both had no problems putting a full 20 round mag onto an 8x10 steel target at 400 meters.
Cons: Odd balance, the movement of the internal mechanism while firing was annoying to me. I don't know how to explain it other than to say it was really annoying. The 91 will kick out the brass remarkably well, in fact it does so with such force that even with an ejection port buffer you won't be reloading your brass from a 91 if that's a concern to you. Stock trigger is as sensitive as the tailgate latch of a 1970 Ford truck, but the trigger group can be replaced with a quality one (albeit for a hefty price.) Like I pointed out about its accuracy above, it's battle worthy but it's not going to be a tack driver without a lot of very expensive work. Of course it'll shoot accurately enough for its intended purpose which, for me, is enough. Parts can be very difficult to come by.
FN FAL
Pros: Reliability is competetive with an AK47. So many Western countries have used the FAL that it's affectionately (and correctly) called the Right Arm of the Free World. The gas adjustable system allows you to decide if your empty cases are going to fall out near the shooter or be tossed into the next county. Parts, especially magazines, are available in droves. There are so many spare parts for the FAL laying around that if you have a lower receiver you can build a whole rifle very easily.
Cons: Accuracy is on par with the H&K, it's certainly capable of delivering minute of kill zone to any range you're going to be reasonably shooting, but you're not likely to win Camp Perry with one. Iron sights are acceptable but not great. Triggers aren't as bad as the H&K and they're much cheaper to work on. Scope mounting on stock FALs isn't great but with the addition of an Extreme Duty DSA cover it's a breeze, and it's solid.
AR10
Pros: Great accuracy with the potential for competition accuracy. My DPMS in .243 Winchester will shoot .75 inch 5 shot groups all day long as long as I let the barrel cool between firings. Ergonomics are great to me, I know this is a subjective category but I love the fit and feel of the AR10. Sights, whether open or scope are fantastic. One of the best things I like about the AR10 is the ability to change calibers by just switching the uppers. With one rifle I can shoot .243 Win, .257 Roberts, .260 Win, 6.5 Creedmore, .270 WSM, 7mm-08, 7mm WSM, .300 Savage, 7.62 NATO, .300WSM, .358 Win, or .450 Marlin. Imagine, one gun that'll let you shoot nearly anything from the .243 to the .450 Marlin. THAT'S versatility. The design is well proven and dependable, finding an armorer who can work on an AR style rifle is VERY easy.
Cons: There are a few compaints about reliability. In my experience many of these are word of mouth 17 times removed, coming originally from guys who carried a first generation M16 in 'Nam before they had the bugs worked out in a climate that even made AK47s jam. To me this is a non-issue. I've put thousands of rounds through my .243 and my .308 AR rifles without a hitch. From what I've seen I'll depend on one of these without a hesitation. I really have a hard time finding fault with an AR10.
I have very little experience with the M1A or M14 clones. I really wanted to get one but went with the FAL instead. I shot a guy's M1A at the range. Nice rifle, definitely a solid feel, but I decided it wasn't what I wanted. It's certainly not a criticism of the rifle, I just decided I preferred the feel of the FAL better. I can understand those who say it's more of a rifleman's rifle. It feels more like a standard "rifle" than the AR, FAL, or H&K. One definite downside with the M1A is scope mounting. I've seen a lot of mickey mouse stuff out there, none of them seem to be anywhere near as good as the AR. When you add in the caliber conversion ability of the AR it's a done deal for me.