I would go with the Onan generator, as well as an inverter/batteries. The key to Onan generator noise is the installation. Install it right with shock mounts where the pipes and wires are not sending vibrations, as well as a resonator, it can be quiet. Install it wrong, and it can be quite noticable.
I personally have a 3000 watt generator... given the choice again, and an Onan isn't possible, I'd go for a set of Honda 2000 watt twins, or a Honda eu7000i, which runs just as quiet as the 2000 watt models, except has a slightly higher wattage, and can eat the entire amount of locked rotor amps coming from an A/C without fussing.