You might as well get the UDM, it's got a little more power. Not a massive difference but it's still very easy to learn how to machine polish with.
The UK version of the UDM won't be released until after Christmas now, it's been delayed yet again because they need to fit the machines with counter weights so the pad is balanced or something. So unless you are willing to wait your going to need to get yourself a transformer. I have the porter cable myself, the next step for me is a rotary. If I was to buy an orbital polisher again I would defiantly go for for the UDM.