I have a Sprinter with an LP generator. Whenever I boondock I carry a gasoline portable for charging the battery because the LP generator eats through the propane really fast and is pretty noisy inside the coach. If you're going to do any serious running of the air conditioner or extended battery charging, get the diesel generator or carry a portable as I do. I cannot run the 15kbtu air conditioner from the portable I use.
On the other hand, for occasional short use, the LP generator wins. Our main use is to run the microwave and to top off the battery. Unlike the diesel version, the LP generator has no issues with old fuel, the fuel pump, or gummed-up fuel system components. It's much more tolerant of not getting its regular exercise, stinks less, costs less, and at least some think makes less noise. I'd expect that the diesel generator has higher repair costs. The answer lies in how you will be using the generator most of the time. Different answer for different owners.