It is X6.

I am sure that X can detect Rikimaru (with some sort of Radar.) Stealth Killing doesn't work on this match, because X will always be on guard. (In Tenchu, you can't stealth someone if he's hostile...)

I am not too sure how much damage Rikimaru could deal to X with the sword...He might have to depend more on Grenades and sticky bombs, et cetera.