I'm not talking about who fits what band better, none of that crap even matters. But for the sake of arguing, I would say that Hetfield could've filled in for Araya a shit ton better than Araya could've for Metallica.
Either way, all Araya does is yell into a microphone whereas Hetfield could actually sing, with melody, and still retain that harsh, raspy sound which makes my man-vagina fucking drip like there's no tomorrow.
Hetfield is considered by many (myself included) to be among one of the best metal vocalists of all time. Araya is hardly, if ever, mentioned in that category, at least I don't see too many people list Araya in one of those 285982359235 different "whos ur fav metull vokillist?!?" threads.
Having said all that, I still like some of the stuff Araya has done, particularly on Slayer's earlier albums. But there's no freakin way I'd ever say Araya is a better vocalist than Hetfield, no way jose.