Colonial Campaigns Club (CCC)

I've been thinking. Is it better to target the highest quality unit in a stack or is it better to target the one with lowest quality rating?

Now lets assume the units involded are pretty much the same in the amount of men.

Since the highest quality unit is the hardest to rout, isn't it best to target it to cause as much casualties as possible to reduce its strength? And after all, if you target the high quality unit, the low quality unit will suffer a morale check anyway if the highest do, right?