Here you can compare specs:
http://www.dpreview.com/products/compare/side-by-side?products=canon_eos1100d&products=nikon_d3100
I'd go with Nikon here, overall better specs because it's newer, don't let the 1100D's announcement year fool you, it's basically a 450D from 2008 with a new name. And the Nikon can shoot 1080p video.
And this is coming from a Canon user.