If you have the option to film in 4:3 aspect ratio then you should.

If you can't film in 4:3 then you should film in 16:9

Handcam

If you're filming Handcam, then you probably have superview enabled.

If you have superview enabled, you you can't film in 4:3

-> So, most Handcam Filmers should film in 16:9

Videographer

You're probably not using superview, so if your camera supports it - then film in 4:3
If your camera can't film in 4:3 with the framerate / quality you need, then 16:9 is ok.

Why?

When you film in 16:9, we use what the GoPro captures for Photo / Video and Insta

If you switch to 4:3 mode, then your GoPro records extra data from the full sensor.

We can use more pixels for the Insta and Photos.

So - Filming in 4:3 gives

• Better Insta Edit

• Better generated photos

So - it's a 'Free' upgrade :)