I'm about to spend about £500, maybe a bit more, on a camera body. I will be using it for bird, feathered, photography with a decent 400mm telephoto lens. Am I better offf buying a say used EOS 1d Mk2 with 8mp or a new say EOS 500D with 12mp or even a 550D with 18mp? Has anyone compared old and new? My main criteria is image quality.