I bought a diesel generator for a few reasons:
1. The fuel lasts far longer than gasoline while being stored.
2. No fussy electronic ignition components to deal with. (EMP?).
3. Diesels are inherently more efficient than gasoline generators as far as power produced per gallon.
4. A properly cared for diesel, due to its heavier construction, will last longer than a gasser.
Now the downside - Diesel generators are far more expensive than gassers and are harder to start when its cold. There are far fewer choices available as compared to gasoline models. They are louder as well (although my "silent" diesel is reasonably quiet).
As far as someone said up thread, if conditions are so bad that you have to worry about the zombies stealing it or taking over your place, it won't make much difference if it's gas or diesel. You'd better have lead, copper and steel available to protect it and your family.
My wife’s friend has a natural gas standby generator at her house, rigged to automatically start if she loses power. Natural gas gets interrupted far less often than electric (and there’s an option to make it also accept propane). You can get a 7kW generator for $1,800, and a 17kW for $3,700 from Home Depot. Sound level is 66db