I really comes down to personal preference and your whole system setup. Looking at benchmarks they're both very close. You can always overclock the ATI card to match up with the NVIDIA SuperClocked card cause the cooler on the ATI card looks like it cools better with less noise.
If you have two or more monitors and want Eyefinity than ATI would probably be better for your setup as it has 2GB V-Ram thus better for higher resolution multiple monitors. If you only use one monitor and don't care about Eyefinity multi-monitor setup (don't plan to get more monitors in the future) then NVIDIA offers PhysX (with certain games) and Cuda (for applications acceleration) plus it comes pre-overclocked.
Also, depending on your motherboard if you plan on getting a Crossfire or SLI setup (two of the same videocards) in the future then I'd go with the NVIDIA card because the blower style cooler works better with multi-videocard setups.
Hope this helps.