I had to buy the X frame grip through S&W when I bought mine. Hogue didn't offer them aftermarket.
They fit my square butt N frame without modification.
Pachmayr's Decelerator grip features a softer rubber that may cushion the recoil a bit more. Downside to that is the softer grip allows the gun to react somewhat inconsistently under recoil and that will reduce the accuracy. Found out about that aspect with my 610 when I wanted a larger grip that fit me better than the STD Hogue grip the factory mounted on it. When I saw how poorly I shot with the decelerator I got on the S&W web store and ordered up two of the 500 Magnum monogrips, one for the 610 and one for the 625 I had on order.