My 91's weighed about the same as my M1A's, the balance is a little different and seems more forward on the 91's.
I never noticed the recoil to be much different, just a different impulse.
I never liked the scope mounts available for the M1A's. They always placed the scope way to high for me and they didnt feel natural to shoot with. Unless there was a cheek piece there was no cheek weld.
I've been thinking of picking up one of the new Ultimak rails for my SOCOM if I keep it, I've grown to really like red dots and while the mount on my SOCOM works, its a little high and not mil spec. I've got a couple of the Ulltimaks on my AK''s, and they work great. Well made and they keep things low.
For the HK's the ARMS claw mount is actually more versatile and usable than the HK's ring mount. It gives you the option of any scope or red dot, and allows them to be mounted lower. With the right scope, the whole thing fits in a pocket. Both mounts repeatably hold zero, and are instant on or off as needed. With the stock design on the HK the need for a cheek pad is pretty much eliminated if you keep the scope down. You just slide your head up the ramp a little and you still get a decent, natural weld. You also still have instant use of the irons too.
The M1A's irons are definitely better for a target rifle, but the HK''s sights are easier and faster to use as a battle sight. You cant miss that big rear notch and the front globe for quick, close shooting, and the post is always instantly available for more precision shooting. You also dont have to remember "clicks", just turn the knob to the number.
I think most all of it is user preference. We probably all tend to like what we learned on just a little more, then again, maybe for some, its all there is to know. I've been lucky and unlucky enough to have owned and shot most of the different combat type rifles at some point in my life and if I had to grab just one, believe it or dont, it would be the lowly AK in 7.62x39. Light, handy, brutally simple to use, and plenty accurate for my needs. I'll still keep one M1 and M1A though.