BYMeidel-
Bnos Chava has taken much more trendy girls in the past year or two because mesilos closed. Its a very with it crowd although they may be solid girls. Check into the last years girls because they changed very recently.
Bnos Sarah girls have much less emphasis on fashion and they are generally a more yeshivish crowd. Some come from yeshivish homes and some became yeshivish on their own. You will probably find diversity in every seminary