What is a better prime lenses for Nikon DSLR 1.8 35MM or 50MM?

Shooting short films, and videos, and what are the differences?

15mm is the difference.

When you use your zoom lens, which is the focal length that you find most useful?

Of those two primes I'd probably go for the 35mm. I'd suggest that you try some of the camera maneuvers you intend to be making with your existing standard zoom. Things that look okay when the camera is still, can look odd when you allow the perspective to change as you shoot!

There's no "better prime lens" for any varied purpose. There are only better lenses for a specific situation. The 35mm will give you a wider field of view than the 50mm. To get the same field of view from the 50mm, you would have to step back. That distance makes a difference in the subject and background. Whether that is a good thing or a bad, is up to the videographer and/or director. Most folks I know, get a quality zoom lens to take advantage of the varying situation. All that said, I would expect the 50mm to be more useful for outdoor situations.

There isn't a "better", only "different".

Set your kit zoom lens at 35 and then at 50 to see the difference.

50mm gives a more pleasing perspective for everyday use.