Here’s my local camera shop’s break down of the range:
http://www.cameraworld.co.uk/goprocompareI have a basic 3 series GoPro, and it’s very good. There’s WiFi on it, so I can use my phone to control it (there’s now an Apple Watch app too - I’ve not tried it yet). Not all of the cameras in the new range have WiFi so if controlling via a phone or a remote is important, get one that has that feature.
There are loads of brackets and things now to fix it to cars, bikes, your dog, and so on. Thankfully, it seems that the action camera market has agreed on a standard mount, so accessories from one should fit others.