This is pretty much turning into a Sin vs Sin debate, so we should probably get ready to move this into another topic.
It's apparent neither were serious in the fight and even if they were, it's likely that Mel would win due to his demonic abilities. Whether it'd be an easy win for Mel though, is up to debate. King was only looking for answers and thus didn't attempt to do any extreme damage to Mel. His attacks scratched Mel's face a couple times, yet he refrained from using Status Promotion on those wounds, even when Mel wasn't prepared for it the first time. Mel, on the other hand, also didn't attempt to really close the gap between himself and King and didn't start using his demon powers until right before their fight was interrupted.