Esther.
Now, if it were between Aster and
Esther, I would definitely pick Aster! It's a flower name, uncommon, not old fashioned but not modern either.
Astor just looks too boyish for a girl IMO.
One question though: Do you pronounce them the same? The way I pronounce them,
Astor would never be mistaken for
Esther.
Astor is dark blue,
Esther is sort of a light yellow glow color. Aster is sort of blue, purple, etc.