I spent a day out west trading back and forth between an M6 and a Summit 600, and, without a doubt in my mind, the M6 is head and shoulders a better machine for playing in the snow. It felt soooo light, easy to throw around, I felt at home on it in 5 minutes. I climbed some SCARY hills, both some long gradual ones that go on for so long you think heaven must be at the top, and some super steep ones, and I plain loved the sled.
The Summit had as much power, but it was horrible to ride. It felt like it weighed twice as much, was really hard to get on it's side unless you were in really deep stuff, and generally did not inspire much confidence.
That said, a friend of mine who was with me much prefered the Rev because it wasn't as tippy. They were both rental units, and after switching back and forth for a while we figured out that he had a slight preference for the Summit, while I greatly prefered the M-Cat.
The M-7 would be a super choice, and I agree with NDMtnsledder that the only other choice out there is the Raw Polaris. Owning a Dragon 121 I have to say that if I was serious about getting a mountain sled today it would be a Dragon RMK, or even the RMK 600 and some mods.