Off the top of my head: Diomedes is the youngest king among the Greek contingent at Troy, while Ajax is a more experienced warrior. Ajax's role in the army is often defensive and he's used to hold the line, whereas Diomedes is more 'lion-like', 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 23. Ajax is defeated by Diomedes in the fight-in-arms at the funeral games. Ajax is in many ways the paragon of fighting-virtue without great divine favour, and this can only take him so far. I hope this helps some.