Actually there is some truth to that - Nikon in its entry level models bumps the color saturation up a bit by default. This can be adjusted with either canon or nikon though, simply +/- 1-2 in 'saturation' on the image properties within the camera.

Personally I'd go for the 450D. However, if the tilt swivel LCD on the D5000 is particularly useful for you, you might want to consider that too.

Neither will take 'better pictures' than the other though, YOU are the one that takes the pictures, not the camera.

*edit - I'd also avoid the D3000 entirely.