These are the same rubber pads used on the AC Hydraulics jacks. They are solid in the center of the pad, about 20mm (3/4" approx) thick. I don't know the Shore hareness (D durometer) number...presumably 50-60. Even if they do tear up over time, they are easily replaceable.

Wood blocks could potentially split, especially on non-flat surfaces causing the car to shift on the stands. Better make sure those blocks are made of Ipe or some other hardwood. DS