Of the two you referenced, I"d choose the Sims for softness of recoil on a 20 ga. But both a good pads.
On bigger guns I use Kick-eze & have never had a issue with one falling apart. They will however, like the Sims, stick to things like carpet a bit.
Last I knew the Kick-eze was still the best, as tested by those who do such. Does not mean there is a huge difference however over other quality products like the ones you are interested in.