I also recommend the UDM, it has more power than the PC.
One obvious shortcoming the Griots has is a non-standard backing plate. It doesn't use a standard BP like the PC and UDM. The Griots BP is held on with screws. What this means is the availability of BP's will be limited, especially if you want to use 4" pads with a 3.5" BP.
Read the following thread for a review of the Griots polisher (skip to post#47 for the final conclusion):
I think I like it !! - Car Care Forums: Meguiar's Online Archive
The switch issue on the UDM has already been addressed and corrected; the switch on the one I have works fine. As for motor problems, I'm not aware of a widespread issue with those. You'll hear more about any problems here from the PC or UDM because very few members use the Griots polisher. I'm not saying it's a terrible unit, but the power and BP issues would personally steer me away from that as a choice.