I'm just going to assume we are comparing them as actors, since that's the only way we can meaningfully compare them.
You can get the core of what is written below from this, without the annoyingly long read.
Summary: Jack Black is mostly just annoying, Will Smith is cool, what does scientology have to do with their acting ability!?
Jack Black is a mystery to me. He can act reasonably well; he proved that in that one British-ie chick flick movie called The Cottage or something like that. But most of the time he doesn't act, he just is Jack Black. And while being Jack Black is funny for a little while, he's kind of like the class clown or Jar Jar Binks. After a little while I just want him to shut up. If he'd act more and class clown less I'd like him. Heck, I like a lot of his movies and his performances. But after the way he single-handedly ruined King Kong, and reviewing his movies (that I liked originally!!) only to realize he's the same character in every one...
Points have to go to Will Smith, who has the virtues of never being supremely annoying and of being a good actor. Yes, he's good at his job. He has skill, and he has pizazz. Jack Black usually only has one character, and that's kid-that-never-grew-up-or-learned-basic-social-skills.
Scientology? Tom Cruise (Mr. ZOMG scientology "scandal" somehow crushed my career) is a good actor too (though he suffers from same-character-itis a lot too), so overall I don't see what scientology has to do with it. People would scream bloody murder and cry "BIGOTS!" if you judged them based on a conversion to Islam, and they'd be right if it didn't affect their acting: Who cares, and why is it any of your/my business what religion they ascribe to if it doesn't compromise their acting? Should I think less of their acting skill if they choose to worship a spaghetti monster or aliens?
I think that not caring gives you cool points for about 20 seconds, until someone puts it in context and realizes that anyone can be rude, disrespectful, or whatever with a straight face. I miss the boat when not caring about how you treat other people makes you badass or requires badassery. If it were about actually doing something you thought was worth doing and not caring what they thought, then yea that's entirely badass. What you're not caring
about is what makes you cool.
I think this topic is a good/interesting idea, and should have a follow-up, with perhaps some of their more talented contemporaries: Heath Ledger vs... ?
Hey, wouldn't want to suggest both contestants.