Again, about the "most-bang-for-your-buck" theme.
Considering each upgrade to military buildings give pitiful extra troops (with few exceptions, like militia quartes > archery range), isn´t it worth to only build level 1 buildings and instead invest a lot on donjons, keeps etc? Specially because that will give you the largest amount of troops, and since more troops is great both for battles and sieges, I don´t see much reason to get large stables instead of small stables plus other 2 buildings. Sure, keeps take a lot of time to build but you can accelerate that a lot with the steward.