Hi Michael,
I've tried both PiMAME and RetroPie. I didn't like either menu. PiMAME has a simple text menu that launches mame4all directly (or advancemame). RetroPie has a more sophisticated menu (emulationstation) that also launches MAME. Neither uses the built-in MAME menu, which itself is pretty awful.
I've finally decided to make my own menu, which I did tonight using Python and the PyGame library. You pick a screenshot of a game and it runs it. Nice and simple for kids.
By the way I had a hard time getting games to run smoothly on the Pi using those two distros, but once I made my own Raspbian-based install and got mame4all-pi from the source, it all worked really well.
https://code.google.com/p/mame4all-pi/