string instruments and band instruments

The rent-to-own band instruments are all full-size. It makes sense to rent-to-own and provides more options for parents and students. String instruments come in fractions of the size of a full-size string instrument so even a very young child can hold the instrument correctly. It is not practical to rent to own an instrument the student will eventually outgrow. 
This is the reason the instruments are divided into two groups It isn't about the music. It is a way to charge customers fairly. 
For some reason, string instruments are billed quarterly and the other instruments are billed monthly. We're sticking with it.