Quote Originally Posted by konny
That depends very much of the situation.

When you have the money to either field for example two units of Phalangitai Deuteroi or one unit of Argyraspides you should always decide for the cheaper, weaker units because two of them will certainly beat one elite unit of the same type. That can be when you are short of money/income per turn or need to field more than one main stack at a time.

When you have enough money or, due to the strategic situation, can limited yourself to one main army, the 20-slots limit becomes more important. In this situation you should always take the better unit, even if the cost-efficinecy isn't as good because of the much higher price with only slightly better stats.
That doesn't apply to units that are too different in strength though - for example, one unit of Spartan hoplites could kick the arses of quite several levy hoplites, also one unit of Gaesatae could take on many, many low-class units