Well that's kind of a tough question.
If you look at Buffy's choice, there are some good things and some bad things (there always are). Some of the good things: she saved the world, she died saving the world, she died a hero, she went to heaven, or a place like it, her soul got rest, everything was perfectly happy, for once, AND, when one slayer dies, another arises, so the whole, 'Buffy died so now there's no slayer besides Faith' thing doesn't work, according to Joss. Some of the bad things: everyone is sad, there's no one for Dawn, there's no one to really protect Dawn the way Buffy could, who would save the world, and who would we watch on the show called 'Buffy the Vampire Slayer' if there was no Buffy? I think Buffy made a good choice when she jumped. It was a very... Buffy-like thing to do, sacrifice yourself to save a girl who's not really your sister.
On the other hand...
I'm not Buffy. If I were in this situation with my sister..... woohooo let's just say it would go differently. When I found out my sister could destroy the world if in the wrong hands, I would off her right away so that could never happen. That way it wouldn't get as far as Buffy let it get. But if it did get that far, then I would push her in. If she started it, she can finish it, even if it's not really her fault. And then I would probably bawl my eyes out. But at least I'm not battling creatures like dragons and things.
In all honesty, I think I'd be too chicken to do either. :rolleyes1