Pros: Propane, otherwise known as Liquefied Petroleum Gas is another fossil fuel. It is clean-burning which means not only less CO2, but less Carbon Monoxide and non-methane hydrocarbon. Which basically translates to plain English as less toxins. Propane is also highly available and is already sold in many stores worldwide for grill-powering purposes. It is also much cheaper and is sold for about an average of $2 per gallon. While that isn't a major difference, it does add up over time. Also, for all the Americans, 90% of propane is domestically sourced in the U.S.
Cons: In other ways however, propane availability is limited. It is not recognized yet as a major player of alternative energy, probably because it's also a fossil fuel, and hence it has the same issue of there being a finite amount and hence it will naturally run out. And hence, because of that there are not many purpose-built engines for propane. And there aren't many conversion places out there, unless you know how to do it yourself. The last issue is that the mileage isn't great, so in total, the price might even out, because even though it's cheaper, you have to refuel more often.