There is a lot of overlap between the sites, both in what questions are on-topic, as well as which users you're likely to see. (The SF&F chat room even gets certain M&TV tags announced as questions happen.) For questions related to science fiction or fantasy movies and television shows, both sites would welcome such questions.
There are a number of differences, though, which may impact where you choose to ask your question:
- SF&F is, obviously, about more than just movies and TV. Any questions about written works of science fiction would go there. But, more importantly, questions about works with more than one medium will often get different results. For example, answers about Game of Thrones or Lord of the Rings on SF&F are more likely to include input from other media on SF&F, while answers of M&TV may not. (I have seen users who explicitly avoid SF&F just for that reason.)
- M&TV is, obviously, about more than just science fiction. Asking about drama, comedy, or other types of movies would only be on-topic there. But, even for sci-fi/fantasy works, if your question is more about the movie/TV medium (SFX, directing techniques, actor information, etc.), you're probably going to have better luck on M&TV. SF&F does handle behind-the-scenes questions about fantasy/sci-fi works, but that's not really the user base that SF&F is targeted at. Also, questions about
"Analysis of content or theme" will probably not go over as well on SF&F as they would on M&TV.
- Slightly different policies. The biggest one here is the spoiler policy. SF&F has a pretty strict policy against spoilers in questions: we encourage the use of spoiler tags when needed. M&TV policy is to avoid spoilers in titles, but not so much in the question body. (It's an interesting question why the two sites differ on this; my intuition is that spoilers about movies and TV have a much shorter half-life, especially with on-demand watching and what not, than spoilers on books that may take weeks to read.)
In general, if your question fits either site, ask it on either site, and you'll probably get an answer, probably from the same people.