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Master/Apprentice battle.

Nihilus gets no Killer technique. (it's not a drain)

All-out duel.

This is a bit silly. We can't determine the level of lightsaber mastery Sion and Nihilus have from a video game.

Well damn, if Nihilus gets no killer technique, then he's useless.

'cept for his numerous other powers he has over Mace.

What numerous powers? Oh you mean the basic force choke? Or TK abilities? How about force lightning that Mace was able to throw back?

Choke, TK above anything Mace can match, the ability to transfer his consciousness to other items and of course keeping people alive all throughout his ship through sheer will.

Except for force choke and TK, his other abilities are irrelevant to a 1 on 1 fight. That's like me saying Sidious had every power imaginable, yet Mace put him on his ass.

Based on evidence alone, Mace and Depa would curbstomp Sion and Nihilus in an all out fight. That said -- this fight is completely unfair if Sion retains his immortality. No matter how hard he'd get ****ed up (and believe me, he would), he'd always return. So I guess the Sith win by default.

I really doubt Sion can put himself back together again a la Deacon Frost if he's cut in half

2nd team wins.

Janus is right we dont have any clue as to how good Nihilus and Sion is in Lightsaber combat. We can only go by Force capabilities, but even then, Sion has not demonstrated any formidable Force technique aside from him being incapable of getting beaten physically(If that is even a Force Technique instead of it as being just his nature). As for Nihilus we know quite a bit:

Nikkolas has already pointed how Nihilus keeps people(who are supposed to be already dead) alive through sheer will. He was also noted to have created his fleet by tearing it from the mass shadows that surrounded Malachor, using nothing but sheer power.

Im inclined to say that the Sith duo would win because of the above reasons. But because of the fact that we know almost nothing of Sion, he could prove to be the Sith team's undoing. Depending on what level of Force and lightsaber mastery he possess, he could potentially be the weakest link here and could probably slow down Nihilus, which would be dangerous especially against opponents such as Mace who dont need to be stronger in the force to beat somebody.

In the end, Its difficult to truly tell the victors due to lack of information on one side. Basically I apologize for not coming to a conclusion after the rant I just gave above.