I know it's quite a controversial theme, which leads to many arguments about advantages or disadvantages of each vehicle, but environmentally-wise an average small-displacement motorcycle (or a scooter) seems to make more sense. Considering the amount of raw materials needed to make the vehicles and all the replacement parts, hydraulic fluids and lube oils required thru the vehicle's predictable operational life, a motorcycle seems to provide a higher overall sustainability. There is also the road surface occupation ratio for passenger hauled, more favorable to the motorcycles.
workaround ideas to discuss among friends: Motorcycles: the most environmentally-suitable vehicles for Diesel-haters