As a general rule I believe the Nikons are better for low light photography. I have a friend with a D5100 and he takes some pretty impressive images at 2000 ISO that exhibit very little noise. The articulating viewscreen is nice, too. That said, I believe the Nikon lenses are a bit pricier than their Canon counterparts. Tough choice. Although if you have the funds to get the D7000 or the 7D, don't bother with the lesser models.
"The $0.99 menu was put on this Earth for a good damn reason." -Weebl