Machon Bnos yehuda (BYA) is also a great seminary.
The program is very thought out, the teachers are very warm and approachable, and they are great role models for the girls. The program is set up in a way where it is easy to make a kesher with a teacher. Each girl has a personal mechaneches that is always available to talk, and if nothing pressing is going on, she will still have a chat with the girl every 3-4 weeks. Also, every three weeks there are discussion groups in a teachers house, which is another great way to make a kesher with a teacher.
There are tons of fun trips, and amazing programs every Rosh Chodesh. For some reason, BYA has gotten the name of being all fun and no work. This is very far from the truth! While there are a lot of opportunities to have fun, there is plenty of work as well, and there is definitely no getting away without putting in some hard work.
As for the girls, there are usually a lot of girls from BYA highschool in America, as well as other high schools around Brooklyn. Then there are plenty of out of towners, and also girls from England, Belgium, Switzerland, and Australia.
I think that BYA is a very well rounded seminary for regular run of the mill Bais Yaakov girls, not too yeshivish/greasy, but definitely not toward the other way either.
Hope this information helps with your decision! Good luck!