Definitely go with the medium. I had an Omni air spin which is the same shape helmet and was between medium and large, the medium fit but was higher up my forehead like the small is on you. The large fit amazingly well until a small crash killed the helmet.
I just got a large ventral air spin through pocs crash replacement (amazing discount and customer service) fits identical to the Omni, plus with the medium you can get a hat under there as well.