In the DVC studio, there is less storage. The boys would need to share the sofa bed; when the sofa bed is out there is a lot less room to move around. Pull down the murphy for your daughter, and people may trip over each other.
In the Standard room, the boys would share a queen, and your daughter would get the daybed. The space is designed around the queen, so the walking around part is a bit better, as is the storage.
Only you can decide what the balance between $ and space is. As well, I'd comment that depending on dates, point rental is by no means a sure thing. There are many more requests to rent into Poly DVC than home resort points (owners at DVC) renting; at 7 months, when any point can book, availability often dries up very fast, especially for popular dates like all of fall, and any holiday weekend.