well it really all depends on where you ride, if you mostly ride trail but do some off trail powder riding as well i suggest the xfire, but if you soley ride powder then i would go with the m6, because the 2 inch paddles dont like the hard pack all that much, really it all comes down to what kind of riding you do.
I would also like to ask what year are you going to buy ?? Because in 06 you can get a Xfire 600 sno pro in green, orange, black. In 07 you cant get them in orange, black, and green w/ out sno pro. In 08 they are the same as 07, but you can only get them in black. 07-08 have reverse on them both. As you can see I only know about the Xfire, i really dont care for a M-sereies sled considering I dont need one. ALL XFIRES ARE GOOD, it just depends which one you want.
Go with the Crossfire. A 600 is also perfect in my opinion, and I'd go with the 07's so you can get reverse. I do mainly off trail riding, and a 1.25" track is all you need. The 2" will get chewed up pretty fast if you do much trail riding. As was said above, if you are a mountain guy, then a M-sled is the answer, but as I have said many times, I wish they made a Crossfire back in 2004. The Fcat and Scat with the EXt were also nice.