Rear Mech Cage Sizes

So im about to buy a new Sram XO rear mech. The dude is the LBS is offering me a cracking deal on a Long Cage cause he has overstock($100). Currenty I have a med cage x9 rear mech.
Whats the story here, what differences am i going to notice.

I bought some new XO twisters to go with the whole set up a nice KMC chain ard XT rear block

The long cage will be fine Adam but I think your existing chain will probably be too short so I think you may have to replace it?