Over the years I have owned and supported both for my clients, however, my experiences have been that Nvidia's boards have shown to be more problematic (i.e. driver crashes, general failures). That's not to say that AMD/ATI boards have been perfect, just less problematic in my experiences.
I go for whatever the best price/performance is at the price range I'm looking for, but tend to favor AMD/ATI, due to my experiences.