There's a thread somewhere that compares the two.
I much prefer the D5100
Some obvious features...larger LCD, live view, video
Other good things are the built in settings...scenes etc
I'd give it 9+/10
Some purists don't like built in scenarios. I love them. I set up the shot,...letting the relevant scene set the camera, then I note those settings, switch to another mode, usually M, key in those.
That allows me to change things, like the film speed (aka ISO) or the shutter speed.
In the past using this with flash, I discovered how to light the whole room when the normal flash just lit the subject.