Out of the four of those I use the knee pads the most. The foot rest slides around my shower and I can't even put enough weight on it to be able to shave my legs, let alone have sex with any weight against it. The dual suction cup isn't bad if you can get it to stay put, mine slides a bit if the shower is too steamy.
I'd go with the knee pads because you can use them for more than just sex acts, and you can put them together and sit on them too.