A month ago I had my D90 and a D7000 side by side. The images out of the D7000 were better than my D90, but not enough to make me switch. Don't have the $$$ right now.
The sensor in, and images out of, the D7000 and the D5100 are the same. Check dpreview.
The one place the D7000 smoked my D90 is in low light. Huge ISO capability. I have the D90 primarily for wildlife and outdoor photography and I crave low light ability.
If you have an older dslr, or are getting your first, the 5100 seems pretty good for the money. It's high ISO performance will bump you 3 or 4 f-stops in low light, maybe more.
Also, an articulating screen is a useful thing to have.
And what swampthing says. A good lens is often more important than the camera it is on and if video is what you really want, the D5100 may disappoint.
The camera is nothing, wait til you start buying lenses.
BTW, if you need telezoom, the D7000 matched up really well with the Nikon AF-S NIKKOR 28-300mm f/3.5-5.6G ED VR Zoom Lens, a lens that I did not like on my D90.
Good luck!