*cough, cough* :-P
Serious answer, though: I asked this back when I was doing research for the fic, and general consensus at the time is that King Hassan would take it. Personally, I don't think we've seen enough of him after he gave up the Grand title and whatever power or abilities that gave him to know - for instance, would his regular Servant self be capable of possibly defeating sun-powered Gawain in close combat? -, but my gut feeling is that he should have a huge advantage if the fight starts with a sneak attack, but Scáthach would probably have an edge in direct combat because of Gate of Skye and her magical abilities in general.