The Urchins have a extra piece on the bottom that helps to steady the skimmer as it sits on the bottom of the sump. The also have a longer piece of tubing to connect the pump to the skimmer. That's about it as far as differences between the Remora and Urchin go.
If you were talking about the difference between the EV series and the Urchin, that's a different animal. The EV's have much larger reaction chambers and bigger injectors. They are much more powerful and even the smallest one needs twice the pump that the Urchin Pro uses. You can go to www.proteinskimmer.com
and get all the details on each model.