I like quality for all the reasons that Pembroke and DaDoo mentioned, but quantity has its benefits too. Quantity is good for military police, flip prevention, and riot suppression. A big stack of chariots could move from conquered town to conquered town and stop the resistance just as well as a big stack of tanks.
What if there's one cavalry in range of one of your towns that's defended by a single rifleman, and all your "quality" units have other things they need to do? Put a warrior in that town too, and even if the cavalry kills the rifleman, he won't take the town.
Are your terrain improvements being destroyed by enemy aircraft or bombardment from the sea? Cover your territory with cheapo units to absorb the damage. Keeps enemies from landing troops on your shores, too.
Yeah, I prefer quality for my main fighting force, but having lots of units, even if they're obsolete, is nice, too.