I use the Marine (blue) version of Sta-Bil. I heard somewhere that it is superior to the regular (red) Sta-Bil. I believe that is true.
I put it in the gasoline cans each and every time before I pump in the gas. I use this treated gas in all my small engines. I even use this treated gas for mixing with oil for my 2-stroke engines.
A few weeks ago, I pulled out my Champion 3500/4000 watt generator after it had been stored for almost 2 ½ years with fuel in the tank (I always shut off the gas and run the carb dry.) I turned on the gas and pulled the starter cord. Somewhat to my surprise, it started on the first pull. (Yes, I know I should be ashamed for not exercising it for so long!)
Probably the most important thing for storing a portable generator is to make sure it is in a dry location where it will not collect condensation.
Wayne