An advantage of the propane is they will start in the cold,, It does not "gum up" fuel lines from not running, and runs very cleanly. You do not have to worry about fuel since the tank is filled for other purposes(usually heat)
A bad part is,,,maybe your propane tank is getting down to needing refilled when the electric goes out,,,,which is usually in the winter, when propane is expensive. If you are using propane for heat,, you get hit hard because the propane gets burnt for the generator and the furnace at the same time. Some of them are fuel hogs! Check that!
I personally, do not use the "auto on" switch. I believe in my case, it wastes fuel. My house is extremely tight and insulated. The electric could go out and my house would be fine when I get home from work and hit the button to get it started. Instead, with the auto on,,it would have run for maybe 8 hours before I get home,,while no load was needed,,, just wasting fuel. If the switch could be programed to run for 1 hour every 4 hours,,,just to renew the heat and refrigerator,,,it would be worth that while I was gone.
I like diesel because it can sit for a long time and the fuel will still be good. I always have diesel around.
I have a brand new 10KW welder/generator that I am going to use as my emergency electric, replacing the 25KW propane genny. My new house has a lighter total electric load then my old one,,,so the smaller genny will work for me. It will be maintained and ready from welding during the year,,,doing double duty as the emerg genny.
I also have a wood stove,, so if elect goes out i may just go to bed and not care when it gets dark!