Hardware is so close it doesn't really matter. ATI's coolers are large, but still less noisy than nVidia, IMO.
My beef is driver quality. nVidia used to have top-flite drivers but after the FX fiasco, their drivers have turned to cheese as they tried to ramp up the speed. My dad has a GeForce 4 and no matter what version of the drivers he uses, it's BSOD's all day with an nVidia DLL as the culprit. Once he goes back to ancient drivers released a couple years ago, all the problems disappear.
ATI's drivers are very, very colid, save for a minor screwup in 4.9 that gave me lots of screen garbage in Photoshop and Explorer.
What ticks me off, though, is ATI's new Control Center. It adds a context item to EVERY menu in the system, which is an enormous pain, and you can't get rid of it. It keeps thousands of records in the registry -- a complete calalog of everything installed on your system from programs to DLLs. That may be useful for diagnostics, but I don't like programs keeping a database of my machine -- especially in the registry, as if it wasn't bloated enough.
If you do go ATI, get the drivers with the Control Panels, not the Control Center. Trust me.
