I don't know. I'm such a huge fan of both of them. They're my two favorite wrestlers in WWE today. So I would love to see it because in my opinion they're both more talented than anybody on the roster. It's so nice to see both are champions and main-eventers and yes, Punk is a main eventer. He might not be closing out the show because WWE won't let him but in some cases, main-eventing isn't always closing out the show. Luckily Sheamus does do this on Smackdown.
Now you're speaking my language, would hope that these two get a GREAT booked feud, I will be rooting for Sheamus of course, but I like both guys regardless so I would just enjoy my dream match. I honestly believe WWE would invest a bit more in Sheamus and book him to prevail depending on the situation. Can't wait for this feud.
I think if you are trying to convince people that CM Punk is the "Best in the World", then defeating Sheamus would be the answer. This would have to be a match ends with Sheamus running out of gas. If they could push the idea that Sheamus seems to be tiring out, or looking more exhausted than Punk it could work. I think these two could tell that story in the ring. Sheamus is surprisingly solid in the ring. However, if the match is booked No disqualification, or is a match in a cage you give the win to Sheamus. It depends on the match type imo. I just can't see Punk defeating Sheamus in a cage, or in a No DQ match.
Punk is the number 2 guy in the company, while Sheamus is number 4 or 5. In a clean match, if there had to be a winner, it would be Punk. Chances are it would just end in a no contest due to heels attacking both of them.
Though Sheamus might win because I think WWE wants to push him as the next "guy" while Punk is just naturally assuming that role over time.