I have both. It depends on what shows you want to watch. Amazon doesn't have a lot of "free" content with the base prime subscription, but they offer streaming a lot of shows that you can't get on Netflix- for a fee. They both also offer free trials.
Amazon's app is different on every device. I have a Roku TV and Amazon's app works well there- in fact, my TV will search across all the apps installed on the TV and give you multiple sources to watch a given show from. Amazon's app is terrible on Android.
Another option with Roku is Sony's new PlayStation Vue, which is more like a traditional cable subscription without the cable.