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Prosumer, enthusiast, advanced amateur. Whatever you call it, there is a class of photographers who are not pros but are serious about picture taking -- committed enough to spend a grand or more on an SLR body and at least several hundred dollars more on lenses. Those shutterbugs recently got two new choices from the big guns of digital photography. In September, Nikon released its veeeeerrrry long-anticipated D90 (a winner in our latest Best of What's New honors). In August, Canon released the entirely expected but very welcome 50D . Technically, these cameras are not direct competitors. The Canon has a magnesium-alloy body, esoteric adjustments such as vignette correction, and a higher price tag (about $1,200 online); the Nikon is around $900. But after lugging each of them around for the past several weeks, I think they're quite comparable. So the question is, which one should you get? Image Quality If you're this serious about photography, quality is mainly what you ca...