Metacaulk putty pads vs cover guards for electical boxes

I've heard about using Metacaulk putty pads, but just saw that Metacaulk also has a Cover Guard product that just goes in the inside of the faceplate, rather than requiring the puttying on the back of the box. Has anyone used these Cover Guards and found them to be as good as the Putty Pads. The Metacaulk site rates the Putty Pads at STC 49, and the Cover Guards at STC 54. So, it seems like the Cover Guards would be better and easier to use. But, I'd appreciate hearing from anyone with experience with or knowledge about these products. Background: I am building a small studio in my garage (studio framing was just completed this week) and want to make sure that the electrical boxes in the isolating walls are well sealed and/or don't undermine the isolation of the wall. These are boxes in the interior walls in a room-in-a-room construction, and the interior walls are framed with 2X6" and will be covered with 3 layers of 5/8" sheetrock on the inside. If I use the Putty Pads, I'll need to put them on before the sheetrock, because I won't have access from behind.

Also, are these 3M Moldable Putty Pads basically the same as the Metacaulk pads, i.e., they'll also work for sound isolation?