This question is much easier to answer than what you can expect: get the 35L!

While both lenses have excellent image quality (the 35L being a bit better), the 24mm field of view (73′) is too large for most video scenes. While it works for stills, when things start to move the bigger than real life field of view creates some weirdness as things are getting closer to the camera (or frame edges).

On the other hand the 35mm feels closer to the perception of the human eye and will make your clip look natural.

I have shot with both and while I loved the feeling one can get from a wide panorama with shallow depth of field, it is too wide for general use.

Note 1: these comments are based on a full frame camera. On a cropped sensor, the 24L is preferable since the field of view becomes the equivalent of a 38mm.

Note 2: I am not saying the 24L is not a good lens. It has its uses, but given the choice between the 2 for video, the 35L will get more used.

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