What is the difference in a med cage and long cage rear derailleur aside from the physical length? Where would you use the long as opposed to the med?
No real difference other than the length of the cage. If you have three rings and a big cassette, you need a long cage to wrangle all that chain. To optimize shifting and minimize wayward chain syndrome (WCS), you want your chain as short as possible. You can roll a shorter chain if you’re rocking a single ring, a tighter cassette, and/or you really know how to shift.
I was just about to post this answer, then the obsessive compulsive part of me kicked in. So here we go:
|Derailleur||Capacity||What that means|
|Long cage XT||45T||With 22,32,44 rings and 11-34 cassette, you can use any gear combo. Idiot proof, but lots of chain slap.|
|Med cage XT||33T||In the small ring you can’t go any smaller than the 23t cog. Which is fine, because by then you should shift to the middle ring. Slightly better shifting and less WCS.|
I just installed a short cage SRAM X.9 on my Specialized SX. Short chain, clean shifting. Super tasty!