i'm a ditchbanger myself, personally i would go with the M7. i found the Cf to be a pain in the ditches, the torsion bar is a pain in the ass, so is the 1"1/4 lug track. this year i've invested in a 2" challenger track for my sled, removed the torsion bar and threw it in the woods. i should have just picked up a M7.... 2"1/4 lug challenger track, no torsion bar, rear suspension set up for deeper snow. hind sight is 20/20 i guess.
the M's arent that bad on the trails. i'm sure the trail riding you will be doing is going from point A to point B. most likely point A being your house and point B being various offtrail riding areas. distance of a few miles each trip. The M's are a little squirrely on the trail, nothing terrible. and you can't powerslide corners going mach 10... big deal. your ditchbanging and offtrail experience will be ten fold over the CF.
P.S. if your heart is settled on a CF i know where you can get one, possible trade for a M7