Since I brought up the non-electric motor assisted bikes I should mention that both gas and electric have their own sets of Pros & Cons:

Electric:

~ Pro:
----- Quiet
----- Clean
----- Smooth
----- Best for low power builds
----- Less drive complexity to get the motor and pedal drive to work well together and compliment each other
----- Stealth capabilities, in some variations you can run with electric assist without anyone but you knowing its not just pedal power
----- More socially acceptable
----- Better operational efficiency
----- Paying for electricity to charge the batteries is generally cheaper then buying gas even at 150+ miles per gallon

~ Con:
----- Maximum power and/or long range set-ups tend to be heavier then equivalent gas power
----- Recharging batteries takes longer then filling a gas tank and carrying extra gas is a lot lighter weight and easier then extra batteries
----- Initial cost of a quality set-up tend to be higher then a quality gas set-up
----- Have trouble operating in cold weather (winter commuting) and fowl weather (drenching soaking rain)
----- More pollution and environmental impact then a pedal only bike, better then almost every other motorized option option but still a bigger footprint then a pedal only bike



Gas:

~ Pro:
----- Maximum legal power with less weight then electric power
----- Range is nearly infinite, at 100+ miles per gallon a five gallon can can last you for days of continuous riding cross country or weeks and weeks of just regular commuting
----- No waiting for batteries to charge
----- Initial cost of a quality set-up tend to be less then a quality electric set-up
----- Operate better and more reliably in cold weather (winter commuting) and fowl weather (drenching soaking rain).
----- Gas stations are everywhere so you can run on just a tiny super light weight tank and fuel up only at the start and end of a commute of reasonable distance
----- Still way better for the environment then a full sized heavy gas powered vehicle (especially if you use a modern clean burning 4-cycle engine more on that in the con section)
----- Still get your exercise and stay healthy even though you are still burning gas, at least you aren't sitting in a car seat and getting fat
----- If you use a modern, clean burning, high efficiency, 4-cycle engine over all less pollution and less environmental impact that per-person impact of common public mass transit systems and even a full size electric vehicle if you charge it using the standard mix of "grid power" almost anywhere in the world including the U.S.

~ Con:

----- Loud (4-cycle with good muffler alliviates)
----- Dirty (Using a modern, clean burning, high efficiency, 4-cycle engine alliviates) (Crappy old tech 2-cycle aggrivates)
----- Not as smooth of power and can be harder on bicycle drive components
----- Does not work well at all for low power builds
----- Always at least mildly heavy weight, electrics with limited range or low power can be built with less added weight
----- More drive complexity to get the motor and pedal drive to work well together and compliment each other
----- No Stealth capabilities, everyone knows you have a motor and aren't just pedaling
----- Less socially acceptable
----- Often cannot use bike paths
----- Lower operational efficiency
----- Paying for gas even at 150 miles per gallon is more expensive then electricity to charge an electric bikes batteries unless you have very high electricity prices and/or a very powerful electric bike with a big battery
----- More pollution and environmental impact then an electric bike of equivalent power, better then a full size vehicle including an electric full size or even per-person footprint of most public transit but still a bigger footprint then an electric bike of equivalent power



I own, have built, and use both types. I feel that having both available and using the appropriate bike for each situation is the best solution. But then you are talking to a guy with a garage full of bikes some pedal only, and some with motor assist of all different kinds from light fast sleeks to big heavy bulky ugly hulks designed to haul or tow more cargo then most people ever thought was possible.