I've tried them both out of my own curiousity and from my experiences I say: If you are serious about gaming, buy a proper ATi card.
See how that works?
Just because you had problems, it doesnt mean every sinlge other ATi user did/does too.
I had two nVidia cards prior to my 5770, both failed. One fried up, the other one, well I have no idea what happened really, it just stopped working.
However, thats MY experience. It doesnt mean it happens to all nVidia users.
Its all down money/preference. I was skeptical about ATi when I first got it, (mainly because the colours looked crap/out of place), but one driver update and no problems whatsoever, whatever the game.