If you can only have one car I really don't see how this is even a question. I could see narrowing it down to the M3 or the S4 as a daily, but the M3 or the GT500 seems like a lock for the M3.
I think the M3 will be more comfortable for daily use. Also, WITH SNOW TIRES it will likely be more tractable in bad weather.
I also agree with another poster that suggested the S4 may be worth a look depending on your budget.