Consider, if you will, the rambling, incoherant Star Wars franchise. That series has been too long for its own good for years. So its possible for movie series to ramble. James Bond, Alien, Avatar. Where movies are based on a book/comic they tend to be richer stories due to the increased volume of detail in the source material.

Personally I prefer a movie though, because of the consistent ability to tell a story with a guaranteed begining/middle/end.