Looking around on a bike website and different build kits. For the All Mountain build kits and style of riding they recommend a Medium Cage X.9 or SHimano XT GS. And for XC build kits and riding they recommend Long Cage X.9 and Shimano XT SGS.
What is the difference between the two types of derailuers?
Derailleur cage length relates to "capacity", or the derailleur's ability to handle excess chain links (the chain that would otherwise droop when you're shifter to a smaller chainring / cog range).
If you're running a single or double chainring setup, a medium "GS" cage length is a no brainer.
The long cage length ("SGS") provides enough capacity to take up the slack of a typical triple-ring, 27-speed setup.
However, you can run a medium cage length on a 27-speed setup (in most instances) with a restriction: You can't shift to the small/small gear combo without losing chain tension and dropping your chain. That's normally OK, since small/small is an "illegal" cross-chain combo that normally isn't used. However, if you accidentally shift there, the cage will fold back on itself, lose tension, and the chain will quickly derail.
If you don't want to bother with this, go with a long cage setup.
If you want the reduced chain slap, increased chain authority, cripser shifts and greater obstruction & spoke clearance and are ok with the occasional dropped chain if you goof, a medium cage derailleur is a good choice.
Think of the derailleur cage as a second-class lever arm. The pivot end of the arm is the fulcrum. The tensioner spring applies resistance (chain tension) right next to the pivot.
With a long cage derailleur, you've got yourself a long lever arm.
With a short cage derailleur, you've got yourself a short lever arm.
In either case, the tensioner spring is the same, applying the same resistive force to the arm.
You can look at this two ways:
(1) In the case of the long cage, the chain needs to apply less force to the cage (lever arm) to displace the spring. Hit bump, momentum causes chain to move upward against the cage (lever arm), displacing resistance (tensioner spring), allowing chain to hit chainstay. clack clack clack noisey drivetrain.
(2) Imagine playing Whack-a-Mole with two different length sticks. Shorter stick (short cage derailleur) allows you (tensioner spring) to go nuts on the moles (chain) and put them in their place. Longer stick (long cage derailleur) is a little more unwieldy, doesn't let you bang them with as much force, and will get deflected more easily if you don't score a direct hit. About the only thing the longer stick has going for it is its longer reach, and if you have a lot of moles to whack (or a lot of chain links to manage), this extra capacity might prove adventageous compared to a more powerful short stick.