Canon T3i Rebel vs Nikon D5100 for video?

For filming video (short films, ex), which camera is better?

Of the two, only the D5100 can auto-focus in the video mode (not film).

What you may want to know is that if you intend to shoot shorts, you will need more accessories to use ANY dSLR perform as a video camera.

Before you spend any money, do some research and look at the complaints about sensor over heating causing the camera to shut off and something called the jelly effect caused by using CMOS sensors to record video

The best video cameras ARE video cameras. They typically use 3-CCD sensors, NOT CMOS.

For filming video get the t3i is the best on the market right now!
40mb/sec vs nikon 22 mb/sec bit rate, 60 fps in 720p vs 29, 99 nikon fps
the continuous focus of the nikon is a joke, don't be fooled by that.

On the Nikon D5100, movies can be recorded by activating live view and pressing the dedicated record button with the shooting mode dial set to any position. On the Canon Rebel T3i, you have to make sure to set the camera's shooting mode dial to the movie recording position, before you can begin to capture video using the live view one touch movie recording button.