Niggles: Out of curiosity how different are the three alternate stories?
Without giving any spoilers, they're completely different branching storylines if that's what you mean, although the prologue and the first week or so of the Grail War is essentially the same in all three (basically an introduction) after that they go in radically different directions, characters that only appear briefly or not at all in one route are more prominent in another. Fate is the "generic" save-the-world plot which focuses on Saber (the blonde girl) in which the plot unfolds in a more traditional way. UBW focuses on Shirou's character and his internal struggles with idealism and him discovering his hidden magic powers, it focuses more on Tohsaka (the black-haired girl). Heaven's Feel is very different, is more personal and character-driven, also more morally ambiguous and disturbing, it focuses on Sakura (the purple-haired girl).
