Though I haven't used ProPress, I do believe that the fittings have an Oring inside. The press tool crimps the copper fitting against the outer wall of the pipe, making a ridgid connection, unlike the SharkByte fitting, which has some movement to it. This makes it probably a better fitting than a SharkByte, but I still question ProPress in a long term situation. Time will tell.
Lake County, Florida plumber
Springtime Plumbing Services