Ben, the difference between a Medium and a Large is measurement of the length of the backpanel, and maybe even the harness/belt. It's just like buying a t-shirt. I wear a size small, while you might wear a size medium. Even though it says Exos "46", this number is an estimate. The volume will change depending on the size you pick. The size small might only be 43L while a size large may be closer to 50L.
You need to get your torso measured to determine what size will fit you. No don't guess. A person who is 6ft tall, might be a size medium, while a person only 5'9" might be a size large. Some people have long torsos for their height, others have short torsos for their height.
For Osprey packs, their sizing is as follows:
The Medium fits 18"-20.5" torsos
The Large fits torsos 20" and above.
Every pack company measures their sizes differently. For example, I'm a size medium in Osprey and Gregory, a size small in Arcteryx, and sometimes a size large in Golite.
Please do not take this the wrong way, but it seems like a lot of this is new for you, so please make sure you do all your research first before committing. I would say, hold off on the pack, until you have all your other equipment sorted out. Yes $100 is a good deal for an exos, but worse case scenario, you might end up paying 20-30 dollars more a couple months from now. It's worth it to make sure you get the right size pack for your body and your gear.
I would recommend you read this article about sizing, or go to your local REI to get measured