I've only been to Mayfield personally, and a friend that went with me went to YS after I left. I enjoyed Mayfield, you mostly walk in the water/up the river. It's nice and cool and refreshing. We left about 9 and were back by 1 iirc.
My friend on the other hand I think was not really a swimmer and more afraid of being in the water, he preferred YS Falls, sounds like it was less in the water walking. I have YS on the possible to-do list for next time, so I might be able to personally comment on that after then.
AFAIK, I think YS is a bit farther and most usually tie it in with another activity like Black River, Appleton Estate or Pelican Bar.
HTH
~Moose