We have the Official FFXIII walkthrough and it has a bunch of bargraphs and fancy charts. Hope excels in Medic over Vanille because his maximum magic is higher. It suggests these are the best roles for each character:

Commando: Fang
Ravager: Lightning
Snow: Sentinel
Synergist: Sazh/Hope equally
Saboteur: Vanille
Medic: Hope