The MA is always useful option, but you have to have learned MT and have a victorious army. At the time you could first make the HE, you probably do not know MT. HE is not a cut and dried choice. Again, if you are going to do a great deal of fighting, I would build it as soon as I could without a big drain. Often for me this means a leader rush as I will surely find myslef with a leader and not being able to make an army. This is due to the lack of the required number of towns. As to the units issue, I would ignore the Knights as I would already have the HE, if I am building it. If not in the middle ages, I will have a site to crank it out in decent time, well usually anyway. Definetly the early AA units are more valuable than the HE. This is for two reasons: 1- those larger numbers will make a difference 2- I do not need a lot of leaders that early in the game as I will not have enough towns to make many armies. The thing is, you must always consider all the facets and go with what makes sense for this game at this time.