They are implementing the dynamic energy saving scheme so all of their new versions are getting the "E" before the chipset. You know...energy efficient hug some trees and save the planet and stuff.
The P35 version 2.0 does not have the dynamic energy saver. However the P35 version 2.1 does. Since the EP35 is version 2.1 as well, I think they are the same board and GigaByte just messed up the naming scheme on their website.
So a GA-P35C-DS3R version 2.1 = GA-EP35C-DS3R as far as I can tell but version 2.0 is different.