Palas, while it's true that the archer's would give you more points. Phalanx are far better for their purposes. Archers simply don't cut it on defense which is where they belong, and if you have them on offense, you're better off with gunsmen in stead. The very last thing you want is to have a Military score that will be over estimated. Sure you don't want to be underestimated and it may cost you a bit more for a smaller MS, but you sure as hell don't want faulty troops in comparison to what pirates who know what they're doing and will attack regardless of MS score will bring to the table.
However if you wanted to (very temporarily) boost your MS, you might want to choose what gets you the most MS for the least gold. But in the end archer's suck, phalanx are better ;P
It's a very good question.