Seems like cost would be a huge factor, unless these Deeres are particularly cheap and Ms have gotten outrageously expensive.
For plowing snow, I find that weight helps out a lot, and lighter tractors (i.e. newer/smaller) don't stay in place as well against the force of the snow on the blade.
Unless you need power steering to use a loader, or do a lot of serious farm work, I think an M is the way to go - very cool, historic, and lots of power and traction. Cheaper to buy, too.