I have a 3 black and am pleased. I don't have a need to upgrade so I won't, but that smaller 5 looks like it would be comfortable and something I'd like to have over my 3.
I don't know what is available for old stuff, but I have a hero and a hero 3 black. I definitely prefer the 3 because the wifi to my tablet. We can watch the videos during breaks or even hand it to the wife to watch if within a reasonable range. Also, although I haven't used it, the wifi remote syncing to multiple cameras is cool.
I only record at 720p@60. Remember those 4k take a lot of computer to edit and a lot of memory. Storage is inexpensive these days but I don't see a need for my personal use to go above 1080p. I feel that if the things I just said are not concerns, then expense of buying a 5 over a 3 probably isn't a concern either.
Gopro studio is free, so even if you go with a different brand, you have a decent editing program to start with for free.