I suppose just about everyone will build their armies to always contain the strongest, fastest units. Aside from perhaps one, two or three early-game armies, my late-game armies always consist of 4 Modern Armors, elites if available. At one point, some years ago already, I considered building some late-game armies consisting of 3 Modern Armor and either 1 Mech Infantry or 1 Mobile SAM, but thought otherwise of it. The Mech Infantry would slow down the army and I doubted the Mobile SAM would have any noticeable effect at all. Does anyone do anything other than strongest and fastest?