I can tell you a distinct difference between the Pnumeo (road) and the E2 (mtb.)
The Pnumeo is built to maximize airflow in a more head down position as the vents in the back of the helmet are slotted vertically for force air down when in an aero position.
The E2 is vented for a more "heads up" position.
But i know people who use both for both and don’t really have any problems

http://www.bikeforums.net/showthread...ghlight=pneumo