Yes, a few. The biggest difference is that PiPlay includes a web frontend that enables drag and drop loading of game files, as well as controlling different settings.

RetroPie is built off of lib-retro and RetroArch using EmulationStation as a front end.

PiPlay is an sd card image with a custom frontend (configured using yaml) and individually compiled emulators.

You could add many different programs, not just emulators, to the frontend.

-Shea