The most bare bones answer is that the difference between Ann Estelle and Sophie is that each has an entirely different face sculpt and thus, if you look closely, their faces are different (as they are from Georgia's and Gracie's). They share the exact same body.
The more involved answer would be to describe how their face sculpts differ - something easier to answer with a photo. I'll add photos to this post later today. But in the meantime, suffice it to say that Sophie's eyes are larger than Ann Estelle's (Sophie uses a 14mm eyeball and Ann Estelle uses a 12mm). Sophie's eyes (as they came from the factory) are always brown or hazel; Ann Estelle's eyes are always blue. Ann Estelle's eye are set looking towards her left; Sophie looks straight ahead. Ann Estelle has no painted lower eyelashes; Sophie does. Ann Estelle, from the factory, always came with a pair of eye glasses (but obviously these can be worn by Sophie too). Both have the same skin color (with the caveat that the shade varied from lot to lot, year to year). Both have a variety of factory-installed hair colors and hair styles.
Pictures later - I hope this helps!! R.
As Ren points out, this difference in sculpt means they express different personalities. I remember that the one time I went to a doll convention, Claudia and I started asking people we met (who were Ann Estelle fans) what kind of personality each of the dolls had and, as you can imagine, we got a lot of different answers. That's what doll play is all about! I'm not sure how many people are influenced by the Mary Engelbreit definition of Ann Estelle's personality ... but a lot of people seemed to think that Sophie was "sweeter" - more gentle. Oddly enough, this is not the way they are here at Joe Pye Towers. Sophie is a bit bossy and definitely compulsive - Ann Estelle is a sweetie pie (my Annie Laurie, made with the Ann Estelle sculpt, is a sweetie pie too, but also a bit of a rebel who gets under Sophie's skin sometimes.) R.