I'm sure that most of this has been answered in the Phi thread. The basics are the lowered upper deck and the way the roll centers are mounted have been changed. If you were to ask about cross-platform compatibility, the chassis plates and everything that mounts directly to them has changed, the major suspension consumables will work in either car.
I believe that there is a US carpet spec Phi, too...
Speed Merchant Rev7, Tekin, TQ Racing (wire), Team Tamale
RC Excitement - Buy where you race, support your local tracks.