Porter Cable is very easy to use, and well weighted, so most people start with that. Rotarys are more powerfull so can do more corrective work, but in the wrong hands can cause damage, and in untrained hands can leave marks in the paint which looks worse than swirls.
The UDM is a new competitor for the PC, not used one but some people say its not as good as the PC, some say its better, but probably roughly the same. Meguiars are bringing out their own machine fairly soon, and in a UK specification so need to buy a cumbersome and expensive transformer, like you would have to if you bought a PC or UDM, as they use American voltages.