Originally Posted by
Thief
Generally a Mixture is always a good Option. You have FA event that certain units have bonuses when you sacrafice plus your bonuses (+10% air attack) can go up higher in one category later on.
I also like to believe that casualty rates are help spread out when you have less of one particular unit
Finally as for strong against there use to be units with this attribute but i don't think any of the current units have this anymore so i wouldn't worry about it.
Diversify your army but at the same time focus on your strenghts (ie if you have more air attack then focus on air units...just don't make your whole army air)