ATI are at the moment producing better competitively priced products, and I believe run slightly more energy efficient, which does matter when it comes to a PC with a high wattage power supply.
I may be wrong, but I think ATI have the upper hand at the minute on the fastest card too?
I may have to go for a Nvidia card next though for my 3D based work, unless something starts using ATI's tech.
Nvidia has the upper hand in some technology that could be used a lot more later; thats for sure.
CUDA could be used for a lot of things once its potential is unlocked fully.