Id argue that the only thing that really matters is the number of units (specially early on) so unless you can get Longowmen, just get whatever the cheapest thing is (avoid light infantry, so get the one with 400 archers and 100 h infantry) and get as much of it as you possibly can. The differences in morale and other values between different unit types is IMO not worth the difference in retenue cap cost between various retenues types (cavalry ones go above 1200+). It appears that one thing that really matters is the number of units, and if you combine retenue doomstack with the regular levies, and if that doomstack is made of crapload of archers, you may end up winning many battles in skirmish phase alone.