Are you sure you want the x.7? It only comes in long cage.

What kind of bike are you putting it on, and how many chainrings?

Im assuming that you will be putting it on a single ring drivetrain, so the medium cage would be better. It has more clearance from rocks and logs, and it puts more tension on the chain, which is good.

The x.9 is the one that can be found in med cage, and thats why I got it. Im running an x.7 shifter. Single ring setup.