Off the top of my head Diomedes is the youngest king among the Greek contingent at Troy while Ajax is a more experienced warrior Ajaxs role in the army is often defensive and hes used to hold the line whereas Diomedes is more lionlike he goes looking for combat Ajax is consistently referred to as the second greatest warrior at Troy in the Iliad but this is made somewhat confusing in Book  Ajax is defeated by Diomedes in the fightinarms at the funeral games Ajax is in many ways the paragon of fightingvirtue without great divine favour and this can only take him so far I hope this helps some