IMHO the Marathon Winter just isn't that great in snow. Fresh, undisturbed snow isn't so bad as long as it's not too deep but honestly, 23mm slicks aren't so bad in fresh undisturbed snow either as long as they can get down to the pavement.

I'd say Kenda Klondikes, Nokian W106s and 240s are all better but what other folks have said is true. The conditions you describe is not a cakewalk for any of the other tires I mentioned either. I have a rear wheel with a Marathon Winter on it just waiting for the roads to get better because it rolls so much nicer than the 240s I have on my bike now. However the roads are so bad at the moment my speed is more limited by the conditions than the rolling resistance of the tires so I'm sticking with the 240s both front and rear.