The major advantage of Propane engines is that they are supposed to "Burn Cleaner" Most fork lifts used inside food warehouses, for example,a nd other places where the building is closed to fresh air (and animals, and insects, and other contaminates) burn propane for this reason, put a gas engine in such a building and the humans there will quickly succumb to the exhaust fumes.
Just how much difference this makes..... I do not know. But I kjnow that the other night while I was running on generator power I got a exhaust fume alarm in the Motor Home (THankfully I was only using the MH for it's plumbing, we have no other plubming where I work)
Another consideration comes up if you camp in the C*O*L*D!!! parts of the world.... Propane stops working at a higher temperture than gas (The liquid in the tanks will no longer evaporate) I'm thikning the temperture where this happens is a nippy forty below zero, your choice of F or C on that temp, but it might be warmer