I believe Giles would save Buffy.
1. As her Watcher, it's his duty to watch out for her. If you look at his duty as serving humanity, Buffy still wins because she's a great Slayer and has/will save more people and protect humanity. Remember, Buffy's role as the Chosen One means she has a responsibility to destroy monsters but Willow, as the magic user with potential (and only potential) for great power has no such responsibility -- Giles cannot even know that she will be interested in using her witchy powers in the future, and it's not Willow's job to protect humanity.
2. At that point, I don't think Giles really had much experience with "good" witches -- and surely Buffy's remit to destroy monsters included witches (I'm thinking of that evil cheerleader, the one that ended up trapped as a trophy *oops, spoiler warning* wasn't she a witch?), so Giles might weigh the possibility that he'd be saving a potentially evil witch. (Willow go evil? Never happen, amirite LOL) Willow had potential, but potential to do what?
3. As a Watcher, would Giles later go to the Council and say, "Yeah, I let my Slayer die (screw that, actually -- if he drops her he kills her!) because I decided to save this human chick who can use magic." I don't think that would fly.
So, another vote for Buffy Anne Summers!