I am currently using a couple GoPro and a Garmin VIRB 360.
The GoPros are a simple camera to use for recording, but are currently a pain to get GPS data. I have to use an app to extract GPS data and have to use on older version of GoPro Quik 2.5 for it to sync. Not sure why GoPro built a camera with functionality that is not supported by their latest software, but I fell for the latest greatest trap.
The Garmin camera is about the same, fairly simple to record with, but the 360 option is not viewable unless in the VIRB app or uploaded to a YouTube similar app. The 360 video is helpful though, in that the view can be rotated around the camera axis. A little goofy in that you can't see the data overlay when rotated away from forward view.
At least one camera outside, normally on the strut and one in the cockpit. I like to be able to see the prop in my videos, gives me a recognizable perspective when I watch them later. The one in the cockpit is positioned to be able to see my control and flap movements.
I very seldom look at the airspeed during flight, and being able to see what I did afterward is a great help. Both cameras have data templates of varying degrees that allow you to view in the video. The Garmin has a VSI overlay, and along with the Groundspeed, is really helpful in determining aircraft performance. I like being able to see my speed and descent rate after a flight.
By worrying about my airspeed, it was hard for me to focus on my gauges, and hit my landing point. By trying to maintain a Vspeed, I couldn't get slowed down enough when I transitioned to ground effect and the plane would float. By analyzing the video and the data from the cameras, it made it easy for me to see my plane's capabilities. I could apply what Jim was teaching, and what I saw in Patrick Romano's videos to my flying. The data also showed me that I could get the plane in a level attitude and descend toward my touchdown spot with less forward speed and still be safely above stall. Even after all of the slow flight I practiced at "a safe altitude", I found I could slow down even more in ground effect.
You can take all of this with a grain of salt, as I am not an instructor of flying or videography, I'm just passing along what has helped me in the past.