Neither should ever have to be sacrificed for the sake of the other. I'm 100% certain that's been said many, many, many times over the course of this thread but fuck it I'm repeating it anyways because it bloody needs to be said as many times as possible!!
Anyways, that being said I voted for story because I've put up with games that had god awful stories but still enjoyed them because they were fun and had good gameplay. Shit, mario and sonic had next to non existant stories (Just like many other old games) and I still enjoyed the hell out of them. For a more contemporary example, Modern Warfare 2 had a story that was absolutely bat shit retarded and I still enjoyed the campaign. On the flip side, even if a game has an amazing story I won't play it if I don't like the way it plays, and for a fine example of this see Dragon Age: Origins. I thought the sotry and characterization was excellent but I couldn't get into the gameplay.