The best tip I can give you is to try both Nikons and Canons (and any other for that matter) in the shop! Have a good play, change all the different settings, use the camera menus etc and see which one you feel more comfortable with.
The Nikon vs Canon argument is so long and drawn out, but the reality is, there is so little between them all, that the best one is the one you can use! Some people 'get' the Nikon way, other people 'get' the Canon way; Unless you try both, you won't know which is the best for you!
Don't let the sales person steer you towards one brand over the other, he probably just gets more commission.
Also, as Andyjudge says above, have a look for an outgoing model, you may be able to pick up a killer deal on a discontinued model! As a fore warning, Canon will release the 600D and 1100D from March, once they are on the market, the 500D/550D/1000D will all drop in price, so could be very worth while to hold off a couple of weeks!