One will be of a new line and won't be widely available. Every civ will get at least one of the new units. Imperial age upgrade to pikemen. They have more hit points and deal more damage. Their bonus vrs.
Imperial age upgrade to light cav.
They have more hit points and do more damage then light cav. Ten civs will have access to them. Only Aztecs and Mayans can get them. They are weak against knights. They have better line of sight then a scout in dark age, equal in feudal, and inferior in imperial.