those are two cards that are very close in performance. it would be easy to drop links to favor one or the other. both companies did good work with these cards. i would own either one.
I would say it depends where you read the article. If you go to articles and find the article that's called 4870 better than GTX260 so will see the exact same thing, just in the opposite way, where 4870 come as a better choice so I really think in this question will answer, which one you like more ATI or nVidia.
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