I feel that story is a word that's used inappropriately in the modern age. People see the words, "good story" and immediately assume that it means, "good plot". A story is more than plot: you have characters, settings, conflicts, points of view, themes, etc. While Planescape may have a better plot, persay, it doesn't necessarily have a better story (with slight emphasis on -er; it's still an amazingly good story in itself). Take KOTOR, a game with an excellent plot in itself (it's mired in cliche, but the jaw-dropping plot twist you will never see coming rectifies that a fair bit), but it's really sold by its character, its paradigms, and its primary issue with good vs evil. Those are the things that give it a better story than something like Planescape.