For a very long time Nvidia had a clear lead over ATI, both in terms of speed and driver stability. With the FX line, however, ATI took the lead from Nvidia beating any of their cards with the 9800XT and those in the same generation. Not only in speed, but the image quality of nvidia was loosing big time to ati, especially in their AA routines.
Right now, the leader spot is back with Nvidia and their 6800 series. That is both in terms of speed, OpenGL support, and features like SM3. Stability is not an issue with either at this point of the game (IMO), and as for heat, I haven't looked at the normal temp of the latest ATI cards, but my Leadtek 6800 with a very good heatsink runs without any overheating issues event at full load and overclocked.
Based on that my current vote goes to nvidia, but for all we know it can change the next generation. I simply go with what is better at the time.