This type of question can be hard to answer because it depends on so many things: the type of mod, the video editing ability, the attention to detail of the screenshots etc etc.
Try to imagine that both the screenshots and videos are both perfectly created.
Personally, I prefer screenshots because if they are done properly then they can show a lot about the quality without giving too much away.
There’s no doubt that a great video can get me excited about a mod, but I see videos as perfect ways for introducing features and other things that can’t be shown easily in screenshots. Portal 2 is a good example. However, image how much more discussion there could have been if they only released screenshots for the first month. People would have been swapping ideas about the different possibilities. By releasing the videos they showed exactly what the new features were. Of course, Valve know what they are doing and this poll is about MODs not games, but it’s one way of looking at things.
Of course, you could then argue that a mod needs both and that could be true but if I had to only have one, it would be screenshots.
Here’s one way of looking at it. If you see a media update and there are thumbnails to HD screenshots and links to videos, which do you view first?