Well, I think the choices are self-explanatory.
My opinions:
Putting fast units in an army seems wasteful to me, since they retreat anyway, almost duplicating the effect of the army. With armored units, it's even worse because they can't attack multiple times anymore. So, I generally use defensive units in my armies. And since, as a general rule, my cities don't need extra defense, I tend to use them to protect my offensive units, at which role they are very effective.
My opinions:
Putting fast units in an army seems wasteful to me, since they retreat anyway, almost duplicating the effect of the army. With armored units, it's even worse because they can't attack multiple times anymore. So, I generally use defensive units in my armies. And since, as a general rule, my cities don't need extra defense, I tend to use them to protect my offensive units, at which role they are very effective.