I like both. However, the Simms seems to mitigate recoil better, but is more difficult to install. The sticky pad tends to swell and drag with heat during sanding. The soft part of the pad typically remains a little wider than the hard plastic base and the stock. Freezing before grinding helps a little with this problem, but must be repeated as the pad heats up during grinding. The Decel grinds a little easier and usually finishes with a cleaner appearance.
As fish said, it's a bit of a toss up.
