If you have 4K screens you will probably want to do 4K as it look a lot better than 1080. If you do not, then do 1080p@60FPS. Also, 1080p@60FPS is much smoother, so just on that you may want to avoid 4K, not that 30FPS is bad, its just not as smooth. Lastly, depending on if you have the 6s or 6s+ you may want to stay with 1080p. The 6s+ has optical image stabilization at 4K. The 6s only Digital Image Stabilization, but it only works up to 1080p. So unless you have a super steady hand, 4K on the 6s will not look as good as it will on the 6s+.