Brand loyalty is stupid, You should look at them and get the card which suits your needs (or the fastest etc) rather than sticking with ATi or Nvidia...
I got a GTX295 as it was the fastest card, and had the money for it. I also managed to get another 27% boost from overclocking it, which made it more value for money
This time around, I might get ATi depending on how the GTX380 is, but at the moment the GTX380 sounds like a beast... More than twice the sp's of a GTX285, should put it to 50%+ faster in games and past the HD5870 by quite a margin. As long as the price remains competitive though...