She was the main one, Sabe who was the queen's decoy.
I read an interview that said Natalie Portman's mother came to the set, and gave Kierra Knightley a hug, thinking it was her daughter because at the time they looked so much alike.
As for the question, I don't really know.
Kierra Knightley is a good actress, and maybe she would have been able to do more with the role than Portman.
I think she's prettier, but only slightly. (not that it matters much)
Jeez, had the roles been reversed, and Portman was the handmaiden, Kierra would have been a co-star in 2 of the biggest trilogies out in the last 5 or so years.