My winter commute is seven miles each way. I recently switched from 700x28 Conti GP 4 Seasons to 700x35 Schwalbe Marathon Supremes. I am genuinely surprised how much difference it is making in my speed, particularly when I don't over-inflate the new tires. At ~65 psi, I'd say my average speed has been about 1.5 to 2 mph slower, and I'm definitely feeling it more in the quads. On the flip side, when I took my road bike out this weekend with its 700x25's, I couldn't believe how much harsher the ride was.

Anyway, more to the point, even though my new tires have apparently slowed me down, the ride only takes around two minutes longer. I figure I'm gaining traction, visibility (reflective sidewalls) and comfort while also most likely getting a better workout (so maybe my fitness will suck a little less next CX season). I expect you'd see similar results by switching to a mountain bike, maybe a bit more slow down if you use a squishy fork.

I think the bigger difference would be in riding position. I don't really like flat bars and so I've never been happy commuting on a mountain bike without at least long bar ends. I just prefer drop bars for comfort. Wide tires are definitely nice, though.