You can get AOSP on a lot of phones, you just have to be comfortable flashing a custom ROM.
I don't understand why anyone would pay iPhone prices for an Android phone. Even if the hardware quality is similar (which it won't be, because Apple SoCs are at least a generation ahead of everyone else) Android app quality is much worse.
I'm saying this as someone who has only ever used Android.
I just can't see the justification to spending this much money on a phone Google will drop support for after 2 years, in addition to never fixing some issues that are present at launch (look up the Nexus 4 camera reset issue, which Google never fixed).
I'm using a Xiaomi Redmi 2 I picked up new from AliExpress for $125 USD with free shipping to Europe, and I'm running Marshmallow via CM. Works perfectly, in fact most of the time it's better than $300-400 phones thanks to not being loaded to the gills with Google Apps crapware (seriously I have no use for Google Play Music, Books, News, etc)
Edit: for people down voting this, could you explain why? I've left my opinion here and if you disagree with it, I'd love to hear why. I don't believe I've stated anything factually incorrect.