I wouldn't recommend moving a piano across any floor, even with Magic Sliders. If you can't lift it, put some heavy rug material under the wheels and slide on that.
As far as regular furniture, I have found that the felt pads work the best. Assuming you are moving the items across the floor, like sliding a dining room chair. The Magic Sliders can get grit under them and scratch the floor. If it's just sitting in one place, the rubber cups probably work best. Of course, putting down an area rug is even better.
"Good judgment comes from experience. Experience comes from bad judgement." - Will Rogers
DW and I - FIREd at 50 (7/06), living off assets