Costco or Sam's Club Gas?

[Perfectlap]

lately I've been getting into the habit of putting half a tank in when I go shopping at the warehouse stores. I started thinking about the number of times where I bought gas from Exxon, Shell or BP. I could only think of the rare instances when I was low and about to hit a highway away from home. Which got me wondering if saving three bucks is worth going with econo gas. But really its just the convenience of killing two birds with one stone when shopping at these places.

Does anyone else regluarly fill up at Costco/Sam's? Is there really any difference at all in their premium gas?