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?
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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.