Luigi's fast enough to tag Sans, and one hit is all it takes. Luigi's also durable enough and innocent enough to last long enough to MAKE that one attack that matters. Furthermore, every act Luigi has done was for the safety of his home and planet. We can get into the morality of killing creatures to save others, but I doubt KR will affect Luigi that much. Other than that, MAYBE Luigi will be afraid of Sans, but that's not a massive advantage.
Luigi stomps both rounds. By round 2, he'd be prepared and aware of his opponent's weaknesses, while Sans can't really exploit anything.