Size of the library matters, but freshness matters more. An app carrying 50,000 titles that never updates is less valuable than one with just 20,000 that adds new releases each week. Seek out apps hat receive the latest movies soon after their theater runs, and keep TV shows up to date with new episodes.
If you like variety, that has impacts on your longterm satisfaction as well. If all you watch is action movies, a more focused library suits just fine. But most want a variety for different moods — comedies, dramas, documentaries, foreign films. Look to see if they offer the types of content you actually watch, not just big numbers.