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.
"I know rocks. Rocks are hard."