In reply to Alan M:
Alan,
I went through the same considerations a few months ago. I opted for the 5 (which was also available at an extra discount at the time).
I have found in practice that the key features I use are shooting in 1080 at 50fps which allows for decent slow motion clips if needed. Image quality and light reproduction is good. Battery life is decent, and can get a few hours in standby mode while using the voice activation for stop and start recording.
Still images and time lapse work well.
Quick Stories and the GoPro app seem to work OK (iPad and iPhone SE), and the clips are easy to use and work with in iMovie and Premiere.
The stepped zoom is the only practical frustration where the 6 offers a full range zoom.
All the accessories are the same, so no dramas re future proofing on them.
Kev