I have said it before, and I will say it again. Canon. Much easier to use, better build quality. I sold my Rebel XSI when I purchased this car. I am borrowing a Nikon D60 from a friend. The Canon by FAR (much FARTHER), is a better camera out of the box.
The Canon is easier to use, and the kit lense is better.
As far as the Sigma goes, I have used those aswell. Regardless, Canon or Nikon, Get a Canon or Nikon lense. There is a reason they are so cheap. I have used both, on Both Cameras.
Canon Body, Canon lense = Win.