go down to a 2nd hand shop. bring your own memory card and try shooting w a canon and a nikon body 30D/d200 for eg. make sure you mount a comparable lens. ( say 50/1.8 or a 70-200L and a 80-200 ED). DONT compare using a 3rd party ( cos if you are really going into photograophy to the state of which pro use what, its the L lens and the EDs, not 3rd parties).
look at the pics and see which you like. in film days, ppl who like contrasty pics tends to go for nikon. cos the film is the same and it all goes down to the lens. now you can actually dial in the amount of contrast you want. canon pics are softer. not that they arent sharp, just lesser contrast.
once read a nikon user decided to go canon cos he read that canon has 1000/5.6 lens but nikon dont have. or canon has a 5x macro and nikon dont have. so what if nikon have it, are you going to end up buying one cos that is what you wanna do w you photography?
nikon or canon, nikon 12-24, 17-55 and 70-200vr and canons 17-40, 28-70 and 70-200is preactically covers almost everything a amatuer photgraphy will need. both w do you fine.