I have a lowly E320 :-) and I've never had a problem in the snow in Rochester,NY.

I throw on some Pirelli snow tires and I've never been stuck or slid off the road.

There were a couple times I felt the back end coming loose but there was ample warning to slow down and recover. You don't need F1 driver reflexes to recover.

Hope this helps,
Ren

PS I'm keen on getting an E420 also. I drove it in the rain pretty hard and it was nice and stable. I can't imagine it would be worse to drive in the snow as my E320. Yes the engine has more power and lots of passing power in reserve, but if you can avoid the stomping of the accelerator and drive the car smooth you should be fine.