I have both pachmayr decels and sims on my guns. IMO the sims does more due being more cushy then the pachmayr.However Ifind the sims catches on clothing(especially fleece) where as my pachmayrs don't. Seeing your putting it on a 12ga if your planning on wing or trap shooting where faster shouldering/acquires are needed I would be inclined to check a pachmayr grind to fit out. If this is for a slug gun then the sims should be fine.

As mentioned adding weight will reduce felt recoil. I use to fill my savage smokeless MLsynth. stock with 2 small bags of lead shot for shooting robust loads off the bench in the summer. As I was replacing it with a custom wood stock, I didn't want to spend a lot of money on the POS but stillneededit to bea functional ginnie pig.