I would say it probably has fewer movies, but it's selection is more mainstream. However it's model is quite different from netflix's (except if you have a paid for Prime account, with some titles).
The free prime streaming is only a subset of their catalog. It's also confusing. You'll click through to a movie and it will give you options to buy, rent in HD or stream with prime for free.
If you've paid for the account (in other words, it's not a free Amazon Student, or Amazon Mom account), then some of the titles will be available for free streaming.