Out of all the posts you'll ever find in any forum, the only ones which make the most logical sense are those which suggest (as was above):
1. Making sure the fit is right. This not only means that it should fit your hand very comfortably, but that the controls and menu system make logical sense to YOU. I personally find Canon controls and menu systems illogical and far too deeply situated.
2. Budget. While I am a Nikon guy. I am because I thought it was worth it to save for good glass, in order to stick with the other qualities that I bought it for, which boil down to great low light, high ISO performance, ergonomics, and an all round better flash system than the competition.
More to the point though... I wish Nikon glass were cheaper. Canon glass is priced far more reasonably IMO. Granted, Nikon make all their own glass, and don't have their hands in as many consumer based products as Canon does but still... they need to lower their prices if they want to compete for the same audience. Maybe they don't ?
I think all the rest has been said. Go to a professional shop and get your hands on a few different models. And do NOT let a salesman form opions for you in any way, shape or form. If you get somebody who says .. sorry, we don't like dealing with brand X or Y because yada yada.. you won't get a straight answer out of them, so don't bother.
Doug