I can give that answer because I took the free trial, finally. Amazon's selection is good, with fewer movies, though the tv section is very similar to that of Netflix. Regardless, it seems like the only videos that have closed captions are the foreign language movies with English subtitling.
As much as I want Amazon to compete with Netflix, the differences are:
- better search options on Amazon Instant, allowing you to search through genres - or the whole Instant catalogue - by average review score, popularity, alphabetical order, and release date.
- more movies and shows available on Netflix.
- more viewing options, like captions, on Netflix.
- Amazon doesn't remove reviewer data, so you can trust the opinion of reviewers you agree with.
- Amazon has no app to run on devices, but you can play it with anything that has a browser and flash.
- Amazon offers many shows in both HD and standard format.
- the Netflix app has finally allowed you to select individual episodes in order to see the story descriptions for each. Unlike before, you can know what you're about to watch.
For the moment, that's all I have on the topic.