Military cargo towers and barracks (by barracks I refer to the white HM building that you might be familiar with, has lots of rooms) all have the same upgrades.
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These are the upgrades for both of them buildings, they're exactly the same. I imagine the benefit of having a military tower is that it is a highly defensible position but I've yet to see ours in action. You are unable to get any vehicles out at either cargo tower or barracks at the moment but hopefully this changes in the future.
(Ignore the no's I ripped this from a gang database, the red no's just indicate that this gang has yet to buy these upgrades not if the upgrades are not available.)
(EDIT) Also dependant on what type of group you and your friends are in the type of clothing and weapon store changes. If you are a rebel gang it because rebel clothing and weaponry while businesses get civilian weapons, this means only pistols for £15 million and clothing.