I've been watching Fate Stay Night and Unlimited Blade Works (movie). Haven't seen the manga yet.
In Fate Stay Night, Saber is determined to obtain the Holy Grail. After many experiences with Shirou, he convinces her that it is pointless, and so she accepts to destroy it. Shirou convinced her mainly because he wanted her to stay and have a normal life.
In the Unlimited Blade Works movie, there's no hint of major affection from Shirou towards Saber (who seems to be more interested in Rin). In fact, so many things happen so quickly that Saber even ends up as Rin's servant (so Shirou's relationship with her was like... null).
When the Grail is being summoned by Gilgamesh in Unlimited Blade Works, Saber doesn't even hesitate about destroying the Grail.
So in the Unlimited Blade Works movie, exactly what convinced Saber to destroy it?
I didn't really get UBW's plot very well though. I'm not sure if there was something especially wrong with that Grail that made it useless for Saber. Was there?