Again, I have no idea what "A Little Princess" is. I always feel so culturally inept around here. :P
I prefer Sarah, I think. I like Sara, too, but I'm just more used to Sarah. It is pretty common, but still usable. Most Sarahs I've met spelt their name with the H. In fact, I know only one that spells it without it and one Zahra. All in all, I probably know around ten.
A lot of people would probably spell it Sarah upon hearing it first, but I think Sara is gaining popularity. I think Sarah will stay in the top 100 for some time to come yet. I can't imagine it becoming totally unpopular.
I definitely like Sara(h) better than Sarai and Zara. Sarai is all right but too biblical for me. Zara seems like you noticed that Sara(h) is so popular wanted something different.