[Picus]

My father has a B6 S4, I've been to the track with it on a number of occasions. If you're really going to track ~10+ times a year I'd highly recommend the M3. With winter tires you'll be fine in the snow (I drive a 400+ hp BMW in the snow with winters). The S4 is ok on the track, with the grip being the best part. The steering is light and inaccurate, though, and the center of gravity is high. The car "feels" heavy, whereas the M3 "feels" more nimble, likely because of the stiffer suspension, better weight balance, and much better steering. The oversteer of the M3 is so predictable it (imo) far outweighs any benefits AWD offer on a track.

As for mods, being as they are all naturally aspirated you're options for power are pretty limited (aside from FI or a blower). In terms of suspension the M3 has more options, actually it has more options in general; but neither will be particularly mod friendly on the power front.