Personally I’d go for Charlotte. It matches Elizabeth better in my mind. It just seems classic and elegant in the same way as Elizabeth. Both being royal family names might help in that regard. It might also be that my first initial is E and my sibling’s is C. Also, I’m not saying it’ll be the case for your kids, but I’d be afraid that naming them E and E would make them feel too much like they’re a matching set. I know my mother still cares at over 50 years old that her parents named her a nickname for her sister’s name (I never put that together before she saying it, but both start with the same letter). Naming them distinct but compatible names is what I’d personally do. Finally, I think Charlotte allows for more nicknames that can allow your daughter to express herself should she decide. As someone named Emily, I can say that the nicknames are really limited. I wish my name was a bit more unique at times, especially when there are literally five girls at my college with the exact same first and last name. Em and Emmy are nicknames I really only feel comfortable being used by my mother, grandparents, and friends who have known me since we were small children. My mom has started using Mimi recently, which is a change of pace. Beyond that, my nicknames have always just been my initials. I think the same would probably be true for Emma.