This has been discussed a bunch on RV.net. From what I have seen, propane means a built in unit (e.g. Onan), and those that have propane seem happy. Those that don't say it will suck your propane tanks dry in no time.
I have generator prep on my Kingston, and have decided that if I do get a generator, and if I don't go portable, then I will do propane (all the IFs have to do with $$).
Camped next to a trailer in Borrego Springs, CA that had a portable Yamaha 2000W generator with an adaptor to run off the external propane tanks. Thought it was kinda cool as you didn't have to deal with gasoline. Owner was new to camping and didn 't yet know how much propane the generator would use.
Ours is a 5.5k Onan LPG. It is a fuel hog, I have to draw out of both tanks at once or the safety shuts the tank off. If we were running it all day, it might be an issue, but for how much it is used, no problem. Not having another fuel to deal with is well worth it, and I believe it is quieter than the gasoline model.
I have an onan 5.5 gas and am very happy with it. I believe it was always easier to find a gas station than it was to find a propane fill up. The camp grounds we go to did not always have a propane station. I was also told that propane does go faster and it was recommended that I add 2-3 additional propane tanks to go along with the 2 that were already there. That adds weight, which is always an issue, which led me into choosing the gas. I have a 28 gallon fuel station that I use (which also adds weight), but I use it for many things other than the generator. If the fuel station is full, and I run the generator 24 hours, I can get about 4 days out of the tank.