I bought the aimpoint gooseneck and railgrabber mount, to mount my M2. I bought them over the arms versions, due to cost. They however work for me, and have held zero as far as I can tell. You have to realize that this is a red dot sight, minor changes in zero will really have no great effect. The gooseneck mount fits in the carry handle snuggly with no side to side play. There are little tabs on the mount that straddle each side of the carry handle at the front to keep it from getting twisted. Granted the tabs are pretty small and would not hold up to a lot of rough treatment. The play in the mount fore and aft is fairly small, less than a 1/16 of an inch. As long at you keep the thumbscrew tight, the gooseneck mount will not move around on you.
The railgrabber mount uses a torque limited nut to tighten it to the gooseneck mount, so that zero will not be affected. At the ranges a red dot sight will be used, what very small compliance there may be in the system, it has not noticeably affected my zero. One possible drawback to the railgrabber mount, is that the torque limited knob stick out kind of far, much more than a throw lever on a arms mount. I haven't had any problems with it snagging on things, but it does look kind of odd.
I was able to successfully mount my gooseneck mount over M4 handguards. There isn't much of a gap between them, but it fits perfectly. Even if it doesn't fit, it shouldn't take too much to dremel off to get it to fit.
It comes down to this, if you plan on dropping your rifle on the ground repeatedly, using it as a club, or just beating the hell out of for whatever reason, get the arms setup, the aimpoint setup will probably not hold up as well. If you want to have a slick, high-speed, setup that you can show off to your friends, get the arms setup. If your like me, who doesn't care about looks, who transports his rifle in a case, and keeps it slung on a 3 point sling when in use, the chance that it gets hit hard is pretty slim, in which case aimpoint setup will work fine.