I would suggest you consider Nikon product as Nikon can have more sharp image. I have a 450D and image always not sharp as Nikon. Go DCview.com to find out two machines' different result there. You can easy find a lot of photos from this two systems.
要落英文就來吧... Regarding your comment about the sharpness of 450D, there're a few threads here discussing this issue. The general consensus is that the default setting is a bit too soft, so the only thing you need to do is to change the default "sharpness" setting from 3 to 4 or 5. As for Nikon, being a user for years, their way of sharpness is sometimes "hard" or "rigid" to me, not to mention the color nature is quite different from Canon (depends on which kind you prefer). But I would say "sharpness" alone is not a problem to my 450D at all.