[uhftx]

Personally I would never opt for an on board generator. The propane usage is through the roof. Gas is available 24/7 in most areas. Propane tanks expire after 12 years. You still have to do oil changes. Why deal with the extra weight in the front compartment.
The extension cord 20/30/50 amp depending on generator capacity is a minor inconvenience plus it is quasi portable depending on your predicament / setup, portable/or semi perm mount in the bed of the truck. It is useful at home when you're not camping or while helping someone in a field or near a stream for power tools.
Why limit yourself? It is ultimately your decision. And my opinion is mine but yours may differ and that is OK too. Everyone is different. I'm sure you have a reason for asking. Just weight external input against your lifestyle and make an informed decision.