Deciding between Nikon and Canon is like choosing a religion: both are top notch / state-of-the-art systems, and deciding which is better is just debatable. I use Nikon, but if I had to do it all over again I may go with Canon, as I feel that their lenses are better value.

But there's also nothing wrong with the Pentax, Olympus, Sony etc DSLRs either.

The issue with buying used is, image sensor technology is advancing so rapidly in terms of noise reduction, dynamic range, and so on, that brand new cameras are just a big step up from two year old cameras. Still, for a first camera, prob doesn't make much difference.