Yep, let's face it:
Sion would have been killed at least a couple of times in TSL if he wasn't immortal.
-Death on Harbinger
-Death when challenging Nihilus (cut content?)
-Death by Atton (cut content)
-Bested by exile (final death)
Not really that good, imo.
Seen Revan's total power and position in the Jedi Order (he knows about the Jedi covenant in the comics, which was the elite of the order) I'd guess he'd win hands down.
"I think the Devil ****s Dutchmen."