I guess it comes down to personal taste and what you plan to do with the video.. 4K is the sharpest and gives you the most latitude if you ever plan to crop while editing. It also looks great if you have a wide screen 4K TV that can read thumb drives.
I'm happy with 1080P for general flying around the neighborhood. Plus on the M2 Zoom, using 1080P permits that amazing 4X zoom. (You only have 2X zoom at higher resolutions). For anything special I use 4K