I prefer Canon to Nikon, and have had both, but I know its a personal pref thing. They both have superior lenses, and while I respect the Nikons nikor lenses, I find that the electronics end is where Canon really pulls off a win in digital SLRs. The EF lenses are just easier for anyone to pick up and make awesome looking photos... with the Nikon... not as easy. The lenses and such, yeah, Nikon might be great, but the sensor end (CCD, focus, controls, etc) is where I feel the Canon comes out ahead.
Besides, every Nikon I have had seems to drop in price so fast... the Canons seem to hold their value for resale much better. I sold my 20d and bought the 30d for only a few hundred dollars... If that had been a Nikon, I might as well just bought the whole new camera. The last Nik I had went from $1500 to $600 in a year.