D5100's price above seems higher than the current selling price.
When getting a DSLR, you'll be getting into picking a "system", not just a camera. That means not just one but possibly more lenses depending on your shooting needs, flashes and other accessories. Both Canon and Nikon offer comprehensive array of photographic system for evey budget and need.
Picture quality and capabilites from both systems are about equal for all intents and purposes. I recommend that you go to a seller that carries both lines and check out both cameras to see how each feels in your hand and see how the button layout and the user interface (UI) appeals to you. It becomes a matter of personal preference; in the end, you can't go wrong with either system.
If you are coming from L120, Nikon's UI maybe more familiar to you.
For action shots and AF speed, you may have to consider two things. Ability of the camera to shoot at higher frame rate and use of fast, constant aperture lens (f stop rating of 1.4 -2.8). Both adds in some cases, considerable cost to overall cost of the system. You may have to consider higher cost, more capable DSLR body such as Canon 7D or Nikon D7000 if actions shots and speed is a paramount concern. Also consider investing in fast prime or zoom lenses (prime is usally cheaper but may not be versatile enough).
