in game terms (n RSII stats not IRL)
Levy Phalangites = low level frickin militia phalanxes who can hold other troops at bay, but will be massacred against another phalangites
Katoikiai Phalangites = medium level militia phalanxes who can stand their ground better
Pezhetairoi Phalangites = regular phalanxes, who are simply better Katoikiai Phalangites, their skins are cooler too...
Argyraspidai = frickin elite phalanxes, why? because everyone need expensive toys for playing in mid-late game when they become empire proper.