Add me to the list of buying the real thing.
I started out with a AKASO Brave 4K ($80 on sale). Decent reviews, awesome price for 4K.
I do a lot of underwater video so tried it that way first. Always looked washed out, etc. My only reference at the time was my Sony A7rII in 4K. The image quality of the knock-off action camera was so just soft, washed out, etc. So I bought a Hero 5 black. Just one video clip and there was no going back. I thought I would use the knock off for mounting outside the plane but I think I will even skip that.
Watch for sales on Gopro's, often new ones will be like $50 cheaper. Also, the ones in plain box (not retail boxes) are cheaper. Definitely check used like craigslist. I now have (3) Hero 5 Black's and the last one only cost $125 with 2 batteries...wish I would have started that way.
I have three gripes about the Hero 5 Black (but I sure do love the built in display)
1.) Battery life. Don't expect to record a 3hr flight. However I have a few liptstick sized power banks (2800mah) that could probably run it 3hrs between the low power bank and the built in battery. Expect around 1hr give or take at 1920x1080 30p and that's with the display off.
2.) The audio adapter is expensive ($50). I left my last one at the hotel in Rapid City
I think if you buy a Go Pro 4 or earlier the audio adapters can be knock offs.
3.) The Go Pro smart remote is both awesome and frustrating. It took me quite a while to get it to always turn on all cameras, all start at the same time and all turn off together. Now I don't need to record the entire flight and instead end up with maybe just 10 perfectly aligned clips. The downside of not recording it all is that you might miss something interesting on the radio.
I also agree with the FlightFlix mounts. Nice!!!
And get the correct audio adapter so you can hear your Intercom and Radios (I think its "Nflighcam Digital Aircraft Audio Recording Cable" on Amazon).
If you get more into it, you might want to buy a small aluminum housing which allows more options for controlling prop blur, etc.
I think other decent options include the Garmin Virb's and some of the Sony smaller action cameras.
For night flight to see inside and out really, really good you will most likely need a DSLR with a good low light lens. The go pro footage at night can get pretty grainy.