don't know how you are going to use them so i don't know which one would be better, but they both have thier ups and downs.
o'shova cannot have some army choices like kroot, gun drones, or etherals in his army, which is not a good thing (i think there is more but i can't remember), but you can add +1 ws and +1 initiative for the fw in o'shovah's army for +5 points each.
wun'shi costs less to field than o'shovah, and can be killed easier if not careful. although he is and inspiring presence, if killed your troops have to take a morale check.
look for more info in the codex if you don't already have it.