I think those are both great cards. However, which ever one you pick will become obsolete in a matter of days. I've run both NVidia and ATI and i've been pleased with the performance on both cards. As of right now, i'm running 2x 8800GT's SLI and the performance is great.
So it's really personal preference...
If it I had to choose the best based on maximum performance because the ATI could beat the pants off NVidia with some multimedia applications, and totally suck ballz compared to NVidia in others...
However, if i had to choose, i would say at this moment, the ATI card only because there seems to be more support in both AMD and Intel camps. For example: I'm still waiting on an SLI board that's AM3 only with DDR3 support...
AJ