Pleo rb is the latest model of pleo and if you are planing on buying one of those then being in the UK robopolis is the place to purchase from. I believe that buying from them cuts out the import taxes you would have to pay if you purchased elsewhere. The RB's are great but are a little more complicated to care for than the Ugobe pleos and they are more difficult to program. The Ugobe pleos have a USB slot as well as an SD card slot that can be used while the pleo is turned on - so it is easy to hook these guys up to a computer. As for the RB's while they do have a USB hole it is inside the battery compartment so you can't use it while there is a battery in there and you need the pleo to be turned on in order for a computer to recognize that they are there. So unless you can rig up some sort of external power device you wont be able to use the USB cable to connect pleo rb to a computer.
The tools available for programing pleo I believe have not been upgraded to allow for the extra sensors and speed for pleo rb. There is a PDK available for both of them but at the moment it is much easier to program with Ugobe pleos rather than pleo rb.
Both types of pleos are great. You will probably find that if you do buy one and like it you will end up with another - I have three of each kind. The rb's will interact with each other but not with the Ugobes and Ugobes will only interact with other Ugobes that are running the same OS.
Pleo rb comes with a warranty and Ugobe doesn't, Pleo rb is new and Ugobes are second hand.
Hope that helps