I prefer ATI, but they're really buggy now I hear, and supposedly the NVidia is better, but I'd personally say ATI.
I prefer ATI too and I've had about 5 different cards, 3 of which rather new (X850, X1300 and a X1600XT ) . Before you think I broken them all, no I just like to upgrade, I've never had problems with any of them. Not one crashed, no bugs, no lock-ups, no burners.

Not sure why the current ATI cards would be buggy, but I haven't noticed anything about it.
As for the topicstarter's question. Are you planning to play DirectX 10 games? If yes, then I would advise to wait a little longer before you buy a new graphics card. The drivers of the Nvidia 8800 are not so good (none for DirectX 10 at the moment!) and ATI's equivalent supporting DirectX 10 isn't quite released yet. I love the way their PR says 'any minute now', but almost secretly they announce another delay anyways. :p