IMHO doesn't matter much. It's more of the market at the time. Right now I believe that nVidia has the best performer, but it doesn't matter if the majority of consumers don't/can't buy it. It all matters of what is best at your price range, and that's constantly changing.
Overall, they're both about as reliable.
On the subject of Dual video cards I've found over the years that SLI tends to scale a little better than Crossfire, although I haven't owned a crossfire setup (I've done plenty of research though).
Oh and mr.Me II you are correct about image quality. Before the 8xxx series ATI was usually given the award for better image quality, however in recent generations of graphics cards there's not a big difference in quality.
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