depends on what you emphasis more lo.
be warned, i am a noob and what i say may or may not be correct...

but from what i see from various camera's sample pics la,
canon's pics are very sharp and normally retains the sharpness even at high isos. good for sports i think
nikon's pics are more to very vibrant colors. good for scenery and portraits i think.

maybe the color have something to do with their lenses (can anyone clarify this?).
cos i saw pics snapped using leica lenses have one of the best colors i've seen, even though they are using different camera bodies (leica m8 and epson r-d1s).

overall IMO, i think nikon makes better lenses than canon. nikon's microscopes are among the top class in the world with leica. summore, i've seen pictures taken by nikon and canon's 50mm f/1.8 lens. both cost almost the same but nikon's are more natural and creamier than canon's.

i'm just a noob here, if i'm wrong or anything, please correct me cos i want to learn more about this. :-)