I have several interesting comparisons of my own this winter. I live in Chicago so the temps are very cold (between -30F and 25F), but the snow doesn't really accumulate all that much, we never have more than 4-6 inches on the ground at one time. Last year, I drove the Roadster almost every day, with the soft top and Sport tires. Overall I thought it worked amazingly well. I was reasonably careful, but not once did the car ever get out of control. I did turn off TC a few times to get through some deep snow in alleyways, etc. As for the the heating, I thought it was adequate, but not stellar. The heated seats make me sweat if I use them for more than 15 min at a time, no matter what the temp.
This year, things are different as I now use my hardtop and switched to winter tires. In the small sample I've had (two weeks of cold temps and medium snow), I've seen a huge improvement. The car handles better than any car I've ever owned in the snow, including a AWD 3-series BMW I owned. I tried pretty hard this past weekend to push it a few times and still couldn't really get a decent skid going. I do find that the car fogs up inside, and it is pretty hard to get rid of no matter what I do. Also, it could use heated mirrors as well. Those things are more pronounced when you are in a car with limited visibility to begin with. The cabin temp is now much higher with hardtop. I barely use the heater at all after an initial blast.
Overall, I am very pleased with the performance in winter, which works out well for me since I sold my 2nd car this summer...
