Flip a coin. Really, they're both capable of excellent work. At one time or another, one will have an edge over the other but it tends to go back and forth. Over time it all equals out.

I've shot Nikon for several years and don't have much equipment at the moment due to selling some of it, then soaking my DSLR and one lens over the summer. My flash will need to be updated to take full advantage of the current Nikon system. I did look at switching to Canon because I figured this would be the easiest time to do it, but I don't see any major life-changing advantage to doing it - especially with the D300 coming out.