i was in the same predicament as you - except it doesn't snow in Houston enough to make a difference - ever.
I test drove both of them (and the C63 Sedan) and M3 took the cake for me. Way nicer drive and power than the S5. Way better transmission (at the time) than the C63. The S5 does look freaking awesome but not comparable to the M3.
BTW you are asking a bunch of M3 guys if an M3 is better than an Audi. Lol. Kinda biased place to ask.