I need a camera mostly for shooting short videos. Which one is better for this?
I know that the 500D does 1080 at 20fps, but would it make that much of a difference from the 24p in the D3100?
If you're primarily interested in video, the Nikon option offers 1080p and 24 frames per second with autofocus. This exceeds the 500D all around, so if you have no lens investment in Canon, I'd have to advise the D3100.
I'd recommend reading an article from Philip Bloom, a professional filmmaker who uses DSLRs on a daily basis. Some time ago he reviewed a bunch of options http://philipbloom.net/2010/08/19/whichdslr/
I have no experience with Nikons, but for Cannons, if you can, I'd go one model up - 550D. It has 24p and overall is a ridiculously good camera.
Difference between 20p and 24p is pretty big. You won't notice it in a talking head video but as soon as you start moving the camera it 20p does not look smooth. Moderately fast panning will look terrible already.