[Looking this over it kind of turned into a random blurb of thoughts without a coherent structure, but here it is any ways.]

I think ultimately game-play is more important, because you can't have it be a video game without it. Interactivity is the hallmark of the genre, if you want a good story delivered without the game-play then you have other media such as film and text.

Also I think it depends on how hard the game is trying to ride the story angle. In a game like FF13 which sacrificed many elements to achieve tighter narrative control it basically lives or dies by how much you get out of the story. A game like FF1 has a story that is more or less ignorable if you didn't like it but liked the game-play.

Obviously for something like an FF game it is the package of the two together that makes the experience. And the better one gets the more having the other drag it down is noticeable.