My vote is for MTB as you can have better gearing and really fat tires (the key to fun winter riding), and IMHO flat handlebar makes the bike more nimble. I also happen to really like mountain bikes so my opinion may be biased This was my first Winter riding no-matter-what and I had no problems with derailleurs. The trick is to use light oil not grease so it doesn't get sticky and brush it off after messy ride. You just have to re-apply the oil more often, but you have a clean chain. But this applies to both MTB and roadie drivetrains.

Also, I've been told several times that MTB parts are generally tougher than roadie parts.