The MAC (for example) catted h-pipe sounds like more of a treaditional h-pipe. It sounds similiar to the MAC h-pipe without cats, except the catted version is not as deep/loud (slightly). Also, you can expect slightly less of a gain with a catted pipe as opposed to an offroad pipe as a catted pipe restricts the air flow more. The Procahmber has a sound similiar to an H-pipe and an X-pipe. It sounds more like an H-pipe below say 3,000rpm and more like an X-pipe after say, 3,000rpm. It is pretty loud when paired with a 1chamber, or chamberless cat-back. Hope that helps you some. If you want my opinion, go with the Prochamber. Great sounds and good gains!
I had a Pro-Chamber on my 88 GT and actually found it to be quieter than a traditional h-pipe. The sound was awesome however,deeper and more "tuned" than a regular h-pipe. (by tuned I guess I mean not "rappy" like some glasspack equiped cars)