I love the look of Sheamus, the character, and the matches are good. He's already proven himself to be main event worthy, he's a two time champion after a year and a half or so in the WWE. He provides a unique anti-Cena feel, where you just get the feeling that when Sheamus gets in the ring, it's gonna take some serious firepower to put him down. His character feels so natural, and he plays it to perfection.
Now I'm not taking anything away from Wade Barrett. He too has shown he can be in the main event scene, but he needs more time to evolve and develop his character. Yes, he does seem to have a lot more potential, but there's no guarantee that he will rise up, and there's no reason to believe that Sheamus won't improve in the future either. Also, there just seems to be that feeling that when Wade gets in the ring, it's only a matter of time until Nexus comes down and does the work for him. It tells a great story now, but he needs to show that he can really work a match alone, that he's not just an evil mastermind, but an actual threat to anyone one on one.
So based on that, currently, I have to vote for Sheamus. He's proven himself a lot more than Wade Barrett has, even with Nexus's success. I fully expect both to improve, and maybe Wade will surpass Sheamus. But he hasn't done so yet.