As a frame of 1080i is acquired by making two scans of 540 vertical lines from top to bottom of sensor, a moving subject will be captured in two different places between first & second scan. Time elapsed = 1 Frame. The final frame is a composite of the two 540 line scans, interleaved to make 1080 vertical lines, and will show the two positions of the moving subject alternating every line.
A frame of 720p is acquired by making one scan of 720 vertical lines and will capture one image of the subject, moving or not. Time elapsed = 1 Frame. The final frame is exactly as it was captured, 720 vertical lines, one position, subject moving or not.
Because 1080i captures more motion per frame in time than 720p, its not only possible to create slo-mo by just reducing the playback frame rate, it's also possible to get more "frames per second" by utilizing software to interpolate the separate fields of 540 back into 1080.
Which is better? Depends on what you want in the end. The beauty of 720p is that it is what it is, but it isn't full-HD. For the cinematic look, 720p is a natural, in fact 720 24p is perfect if you're going for a film-look, complete with natural motion-blur. But if you want detail and that "reality" look, then 1080 60p would be your best choice, but as the FC200 doesn't offer that you'll have to settle for 1080 60i. It will look crisp, clean, and "live" if you play it back at its original frame rate, but if you're shooting 60i for converting to slo-mo, you'll have to deal with its shortcomings in post-production, esp if there's lots of motion. It takes more time, but the results can be very good.
If you want full-HD (1920x1080) and have no need for slo-mo, then the FC200's 1080 30p setting will probably be the one you'll be most happy with.
iDrone (the Very)