I'm in the in between boot size too. I always get large bindings. Rome, Flow, K2, Burton, Flux, Union, Ride, Forum, Salomon, and Rossignol all fit my 9.5 Burton Hail boots in their large bindings.
I just had to minimize the straps and make sure the gas pedal and heel ramp were adjusted accordingly (when available).
I'm long done growing, but my reasoning for getting a large rather than a medium is because not all boots are low profile like my Hails. This leaves me room for future boots that might be bulkier.
By the way, it might be a good idea to contact Flux and see if you can get the toe straps for a medium size. Going off memory, I recall Flux bindings having fairly wide toe straps.