CM Punk is better than Wade Barrett across the board. Better talker, wrestler, character, leader... Barrett has nothing on Punk. I don't dislike Barrett, not at all, but, he's no CM Punk.
With that being said, the Nexus under Barrett was much, much better, and I think that had a lot to do with the story it told. New guys creating chaos and mayhem, making sure they solidified spots for themselves on the roster. They were looking to dominate the wrestling world, and did so for quite some time. Barrett was the clear star of the stable, but he was one of them as well. Punk isn't.
Punk is a great stable leader. Actually, I think he's superior to Barrett in that role. The S.E.S. was somewhat botched, but Punk shined. His ability to lead a cult-like stable is currently unmatched, and I hope they go back to the S.E.S. eventually, and make it a bigger deal than it was the first time around.
So, in conclusion, the Nexus worked better under Barrett, but Punk is definitely the better leader... if that makes any sense.