Seriously, not much change in the cab robot - the same red and blue color scheme except for the Autobot logo imprinted on the right leg of the G2 Prime.
G1 Optimus Prime (top), G2 Optimus Prime (bottom). Notice the imprinted logo on the leg?
G2 Prime is more heavily armed, now with 2 black missile launchers which fire spring-loaded missiles - a lot more play value as compared to its G1 brother.
G1 Optimus Prime (back), G2 Optimus Prime (front). Distinct difference in trailer color and accessories.