Kriss, we have reviewed both lenses, so you should compare the results page for each.
The Canon's 18-55mm is here
www.cameralabs.com/features/Canon400D_l ... age4.shtml
The Nikon D40's 18-55mm is here:
www.cameralabs.com/reviews/Nikkor_kit_l ... age4.shtml
Remember there are 5 or 6 pages of results for each article, so make sure you click each of the red headings for corner sharpness etc...
I think the Nikon 18-55mm is better than the Canon 18-55mm, but both are budget lenses, so don't expect miracles.