Another post about the warlord versus the military academy versus the military instructor.
The military academy gives a 25% production bonus. But since cities and especially major military production centers usually have a production bonus (forge, civics, small wonders), this bonus is in general not a full 25% (an increase of production from 150% to 175% is for instance only 16.67%). Lets assume that the real bonus is only 15% (over the length of the game).
Each military instructor adds 2 experience to the units being built in the city. But since each promotion costs more experience than the previous promotion, each new instructor (in the same city) has diminishing returns. However, it is interesting to add a few instructors to the same city. If you can get the city to produce units with 10+ xp, then you can get city attack III units or other units with 3 promotions fresh out of the city. This will be an immense help in a war. Depending on the civics that you tend to use and the availability of the Pentagon and West Point, this will cost you 0-4 military instructors.
The next promotion is at 17 xp and I think that that takes too many military instructors. I think it is more interesting to have a few cities which produce 10+ xp units than 1 city which produces 17+ xp units.
When you have constructed a city that can produce 10+xp units, it theoretically could be interesting to get some more units out of that city by using a military academy. However, you can also add military instructors to another city to help that city produce 10+ xp units. As the military academy increases the military unit production only by something like 15%, it is more efficient to add a few military instructors to another city to have 2 cities that can produce 10+ xp units. That way, you will get more 10+ xp units.
The production bonus of the military academy is just too small to make it interesting.
(For the mathematically inclined readers:
If you need x instructors to reach a certain desired experience threshold in a city, then 2x instructors will get you 2 such cities. So x extra instructors give you a 100% bonus in producing units with the desired experience level. So the military academy should give a production bonus of at least 1/x*100% to be an interesting alternative.)
The warlord gives you an instant 20 xp which can be fairly interesting, however after having built 10 units in a city with a military instructor, you will have reached the same 20 xp. And of course every unit after the 10-th that is built in that city will be an extra 2 xp above and beyond what is offered by the warlord. You can use the warlord to create superunits with lots of xp, but even superunits can die and can only kill one unit per turn. So in general, I would also use the warlord to spread the 20 xp over a number of units.
The Warlord experience can be useful to help you when you need some strong forces NOW. If you have an inexperienced war machine in the middle of a war, then it can be more useful to get an experienced war machine now than to get experienced units during the rest of the game. That just depends on how much you need the experience at that moment. It's not a good idea to have a war go on and on while you could have used a warlord to decide the war in your favour.
The extra promotion that the warlord offers can also be interesting. For instance, it is interesting to have a medic III unit in your major attack force (to be used only for healing that group, not for any fighting). You don't want to have your major attack force sitting in cities waiting to be healed when it could also be killing units. The medic III unit can decrease the healing time of your major attack force and thus decrease the time it takes to win a war.
I would generally use the Great General as an instructor, sometimes as a warlord when I'm in dire need for high xp units or when I think a certain special promotion would be extremely useful. I would never use them for a military academy.
