Propane makes you more independent. Natural gas will go away if we have a system-wide breakdown. Diesel is best but more costly for the GenSet. The 1800 rpm ones are better for long term ruggedness. IMHO an auto switch an the least important. When the shtf you will have to be there anyway, you can crank, flip, plug, etc.
I also have solar panels and off-grid capability.
Write if you want an electrical eng perspective.
EE here as well.
A natgas engine will run on propane, but you need a regulator.
The natgas grid will survive for a long time after the elect grid is down, 6 months at least, if there isn’t something actively interfering.
Listen to the Steven Harris interviews on thesurvivalpodcast.com for more really interesting alt-E stuff.