Hi Matt. No-one will be able to tell you for sure just by looks alone. The breeders you spoke to are right however, not all of them are "sqare" looking. Some are more "terrier" looking. Plus the fact she is only 15 weeks old and she has still got some filling out to do.
Don't take too much the vet says seriously. They are great for patching up dogs and giving shots but most don't know too much about specific breeds.
She is very pretty though. Good luck and stick around.
P.S. Some good chew toys will help fill out her cheek muscles but it's mostly genetics.