To answer your question directly I prefer Seafoam over Stabil because it does two functions, stabilizes the gas and cleans the fuel system. In practice I don't add anything to my gas for winter storage. I run the genset several times under load during the winter anyways so I don't drain the carburetor. I use the Seafoam in the spring in a gallon gas tank and run the genset to clean the carburator. Every other year or so I will pour Seafoam down the TBI of the 454 engine and clean it out along with the tow car and add Seafoam to the lawnmower tank to clean it out. I also use it in the crankcase of both engines and am amazed at the carbon buildup that comes out when I drain the oil. Lately I have been using Berryman's B-12 also, it works also and is cheaper. As far as how long is too long if I were going to store gas for two or more years I would use Stabil less Seafoam.
