Grocery store for sure.
The only thing that sucks about the gas station is when you first look at the price for a gallon. Everything else about it is cool. I like the sound of the gas flowing, watching the numbers wiz by is fun. Hell, even if the credit card reader isn't working gas station clerks are usually pleasant to deal with. Oh, and I like seeing that gas gauge go up when you're done.
Grocery store on the other hand is annoying as all hell. The whole experience is super tedious. You can't just zone out in the store, you always have to be engaged. The bill always ends up being high as fvck. After everything is said and done you have to drag the groceries from the store to the car, then from the car, to the crib. After all THAT you have to organize everything. I can't stand the grocery store. Bah!