Both the mercury and the Sims do help control recoil. The muzzle brake does a much better job but has some bad side effects. The 300 RUM is a very loud gun to start with. The muzzle brake will about deafen you and is even worse for anybody beside you.
If you bench shoot for sight in, try putting a sand bag between the butt plate and your shoulder. A heavy coat will help when you are hunting. I'd go with one of the recoil systems and forget about the muzzle brake.