I would say you had an advantage buying a canon camera since you already had one, but when I went from a d40 to a d200, alot of the buttons and stuff changed around so it was like learning it again. I also got a chance to play with a 5d and it only took me a few minutes to learn my way around it. I'm pretty biased so
I'm gonna say nikon but it's really all what you wanna spend.