I'm guessing your investment in lenses is a little dated since you'll have to send one/them back for an upgrade to work with a digital Canon. If that's the case, then get whatever you feel most comfortable with because losing the investment in lenses won't be a negative factor.
You need to be able to answer the questions of why you need the 1/500 flash sync and other sync modes. If you can say for sure why you need them, then buy the Nikon. But remember, Canon will come out with something soon to match/exceed the capabilities of the newly released D40x, that's the way this digital game is played.
I shoot Nikon because I always have so they feel more "normal" or comfortable to me. Get what feels comfortable. One of the most overlooked areas when making a decision is the menu system. This can be very important to an easy/fast workflow so try both and see which makes most sense to you and feels more intuitive.
And remember, you can buy the best body in the world but if you put crappy glass on the front, you'll get crappy images. The most investment should go into your glass.
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