Don't have a 6 or 6 plus yet, but with my DSLR I love shooting 60 fps if only for the option of slowing it down. I love the way it smooths out motion.
Here's some video I shot at my sister's house during a family barbeque. Some of it is normal speed, but a good chunk was slowed down to half speed. It creates a nice cinematic feel.
What would you use 60fps for?
I mostly take videos of my daughters. Any advantage using one or the other?
I leave it on 60fps also. But a lot of it has to do with the option being so out of the way when I want it set to 60. They really should have made the option a button press within the camera app when taking a video.