My personal estimate is that Pit is between 20% and 25% better than AT. Reason? I compared best Pit berserkers with AT sorcs assuming MFO tier of effort (say 30-40 hours) which does influence zerkers more than sorcs in the end. But it is fair to say zerkers are not easy to play, and probably average player can't get most out of him, but nor can he do that for sorc as well, but difference could be a bit less then.
Singers seem to be more popular than zerkers, and they are slightly worse too, but still keeping like ~15% distance between Pit and AT, even for average players. But, AT barbs can be almost comparable to Pit barbs, but are 1. harder to play and 2. no one plays those.
That is the reason why Pit is considered better than AT. Cuz barbs OP.
Also that difference is very small if compared over 20 hours. It is almost not there at all. Almost. Pit barb doing 20 hours is similar to AT sorc doing ~24 hours to give a better idea of comparison. Now how much can 4 hours mean...
EDIT
note that this comparison is between most popular characters in both areas. Comparison is not valid between say Pit javazon and AT fire druid or other builds in general. It depends on the build and on the player accordingly.