From the bolsters on the seat I think that z4 has the regular seats, the sport seats will have bigger side bolsters then that and they will go higher up the seat. I will say z4 all the way it will be more fun then the x5 (which are good on a track, pictured included from one of my recent track days where an instructor took one out for part of a session). As far as concerns with the snow go, I would suggest a good set of winter tires for that, they really make a huge difference in the cars ability in the snow. It won't solve all of your problems but between that and the traction control it should do alright, especially if your located someplace that doesn't get a lot of snow or they attend to it well. If you asked a BMW dealer to run the VIN you could probably find out if it has a Limited slip but I didn't think it was even an option BMW offers anymore (only the Ms get it for the last few years).