Deerman just DO IT!!!
I have always been skeptical about "the latest and greatest recoil pad" that comes along ever couple of years. BUT the Sims/R3 Rem pad is different. I now have 2 R3s that I put on synthetic Rems (one 870 SuperMag and one 700ADL syn) and two Sims (one on a Ruger 77-7mag, one slipover on a 835 Mossberg). IMHO they are THE greatest pad on the market and especially for the bargain price. The only pad I have ever used that is equal is on my Sako/Warbird which features the Pachmayer Decelerator. But those are $100 pads if purchased seperately. The Sims/R3s are easily self-installed in minutes (only takes a Phillips screwdriver and I recommend spraying some RemOil on the tip first!) and then you are done with it.
I likewise have had one of the PAST shoulder pads that I have had for 15 or more years. They are great for the range and target work. But in all honesty any gun that has a Sims/R3 on it doesn't require the PAST pad.
Good luck,
RA