I tried out a pair of both rubber and felt mutes for tuning piano. I concluded that overall rubber mutes are a better choice. These felt mutes do the job, but they are harder to insert effectively (not as flexible as the rubber ones), and do not mute as well as the rubber ones.
One other point to take into account is that being made of felt, they deteriorate with use (I suspect this will only be a problem for intensive use). The threads of fabric slowly disentangle and are pulled away from the wedge.