I use both ATi and nVidia actually... I like nVidia cards because their TV-Out is so much easier to use without screwing around with special cables and adaptors however I find ATi's cards generally more powerful in the real world...
They both produce some very good cards, nVidia gets my vote in the end because their drivers are always much nicer and require less screwing around but I still use ATi for the power... meh, it's nice to mix and match :)
Oh, I might be a little biased since my first proper card was a nVidia GeForce 4 440MX 64MB... no, wait... the TNT2 32MB was my first proper graphics card :D