I am in the same position as you are. The advice I received from someone who has been to both was to do the Mayfield Falls. While both are beautiful, the Ys is more of a hike to a single destination, but Mayfield is a series of falls and you get to walk and splash in the water much of the way to the end.
It sounds like it would be hard to be disappointed with either trip. From the information I've seen, I think that Mayfield is the trip I would enjoy just a little bit more.
Doug & Rebecca
You can contact me through my website : RaynbowsRagdoll
We have only done the YS Falls. We combined it with the Appleton Estates and had a driver for the day. Bring a bathing suit as you can jump into a section of the falls from on top of a small fall or from a rope swing! Water shoes are suggested as the bottom is a bit rocky getting out. (or my feet are just too tender) A YS Falls guide is there to help you with both of these and you should tip him. They also have a zip line for an extra fee. Lots of fun if you haven't tried one. Several nature trails as well. We enjoyed it.