Between the 2, I'd go with Canon 450D. But I'm more partial to canon system and lenses due to preference.
I believe you have read a lot of 'Which DSLR' threads as well.
Some other factors that will affect your choice:
1) camera ergonomics - handling, button positions, handling ease.
2) Lens availability - from friends to borrow/share. I use canon, but most of my friends are using Nikon, so there.
3) Lens choices - original lens choices and 3rd party options.
If you are not concerned about MegaPickles, LiveView and CF card, the 400D is a solid performer for the price. Only 1.5 year old model.