I'm assuming the "get both" suggestions are moot, since Steve is trying to pick 1. So budget is obviously a major consideration. If budget was not an issue, I too would say "get both"

Since money IS an issue, if I was in your situation I would get the X5 and never look back. Wisconsin ain't Florida, so your M3 will be in...how can I put it mildly...a bit of a "rut" for at least 4 months out of the year.
Yes, the X5 won't launch you into orbit the way the new M3 will, but at least it won't launch you into the guard rail when there's a 2-inch snow cover on the concrete.

Also, As rh71 said, beware the 20" wheels for winter driving. If you're in love with the 20s you may want to do the bi-annual spring-fall swap from 18" to 20" and back to 18" with the snow tires. Or if that's too much effort, just get the 19" rims with a premium set of Michelin All-Seasons.

And I'm going to throw 2 wild-cards into the mix: Word from some "sources" is that the X6's twin-turbo 4.4 litre V8 (400hp) is currently making its way into the X5 body. These "super" X-5s may be shipping by this time next year. This is not official yet , but there are spy shots of BMW testing these (on this forum). If you have 1 year's worth of patience that would be another option.

Finally, if you don't have the budget for both an X5 and an M3, but you do have more than what an X5 would cost, the Cayenne Turbo will give you 500 ponies, AWD for tohse icy roads, and a 0-to-60 mph of 4.9 seconds.

Have fun with your decision!