I'd go for the Nikon.
Had the D90 since it came out and have been extremely impressed, particularly with it's 'point and shoot' results. Canons tend to be more the enthusiasts' choice - I like to spend more time finding and taking the shot rather than setting it up, but need the capability of control hence the D90. I find the AF-S 70-300 lens exceptional!
There are good reviews and comparisons
D5000 here and
eos500 here. Both end up with almost identical scores.
Everyone will tend to have their own opinion depending upon what they've got - but in reality both cameras are good: I doubt you would regret buying either!