lighting option may be limited to the plus with dual lenses - tempted but don't want a big, heavy phone. Also you can arse about with lighting in post processing but I dont usually use my phone for portraits..
I think you're fine with the 7, it improves enough over the 6S, while the 8 gives way too little over the 7 to make it worth the extra cash, in my mind.
Unless you really really want that lighting option for the camera, that is.
Is the dual lens awesome though?