I have used both, however currently only use amazon. It depends on what you like to watch. Netflix will never have new releases available to watch. Amazon prime will for a small fee. Both offer old movies for free, but amazon KILLS Netflix when it comes to old tv shows.
I've started to notice that Amazon and Netflix almost have the same movies available. As for tv, Amazon has a bigger selection (old to newer). Many shows can be purchased for a fee. As well as movies. It's really just a matter of preference. Amazon video is available on iPad.