More fuel efficient - probably. Quieter - almost certainly. Easier to steal - you bet. Depending on where you camp, the Onan may be better suited to a Venturi type exhaust so you don't kill the people camped next to you with fumes (has happened). I'll be honest, I've never met an Onan gas generator I liked, I find them noisy and obnoxious to the point where I have picked up and moved if one pulled in next door. At a race track, i usually can't tell if my honda is running or not.
Brian
2005 F250 Supercab, Powerstroke, 5 speed automatic, 3.73 gears.
20 ft race car hauler, Lola T440 Formula Ford, NTM MK4 Sports Racer
1980 MCI MC-5C highway coach conversion
2004 Travelhawk 8' Truck Camper