Joonsup -
Either of the cameras you're considering will give you years of enjoyment. Each is well engineered, but one consideration is which one feels better in your hands. Try each .... have the store put some glass on them and take some photos, paying particular attention to where the adjustment buttons / dials are on each.
I have the XTi .... but have no doubt the Nikon is pretty much a comparable body. That being said, these are higher quality upper end consumer SLR's ... they're not professional quality.
Most important is the GLASS and attachments (Flashes aka Speedlites). The built-in flashes work, but are next to hopeless for better quality shots. As far as the GLASS (lenses) go, if I were to buy either I'd purchase only the body and take the money you save and upgrade to a couple of quality lenses. The better the lens, the better the photos (well ... most often
). I'd stick with NIKON glass for Nikons and CANON glass for the Canon. I've spent almost $5K on glass and assorted accessories, and consider it a sound investment (these lenses will fit on the higher end professional bodies should you wish to upgrade)