I can't decide between these two services, so I ask you what you prefer and why
Depends on. Both of course have different movies / series. So if you are looking for something special you should look where it is. Otherwise you can serve both well. At Prime I like that you can delete respected things again. So they will not be shown under last seen. This is not possible with Netflix. So if you start a series and the stupid is the some time in there. (You can delete it, but that's much more complicated)
otherwise it does not make a big difference to me.
With Netflix, if you have the subscription, you can watch everything there's without paying anything and there are also Netflix originals only available on Netflix.
With Amozon Video you often have to buy the items yourself (borrow) but there's a wider choice.
I would recommend Netflix.
Tendiere more to Prime, especially because for me prime actually no extra investment, because I exploit the shipping advantage and very much for this reason prime member and for me prime video almost as a 'free' addition is considered. If you are more into series production then netflix. But I can imagine that netflix in the next few years even more expensive. Currently, higher contributions in the States are already tested and the prices are then certainly adjusted in this country.