They are both lovely, although I do personally prefer the more simple spelling 'Isabel'. Isabel/Isobel/Isabelle and Isabella have all been superpopular the last few years, though, so she probably wouldn't be the only one in the class if that bothers you. Which goes best with your surname?
I think Isabelle has the potential to date more as Sophie has been popular for a good 20-30 years now whereas Isabelle has only recently got popular. I do actually prefer Isabelle though but they're both great names.
I know loads of little Isabelle/Isabellas, but only 1 Sophie. I'm not sure if it is a regional variation in popularity or just luck. Both seem really popular. I think Pandora is right that Isabelle will date more.
Sophie. Unless your surname begins with S. Or she wouldn't mind people misspelling Isabelle/Isobelle/Isabel/Isobel. Personally just feel as Pandora says Sophie is more classic, plus wouldn't have the spelling niggle.