JHB...I also consider Epstein to be the better GM.
Also, I know that you only looked at what Cashman has done from 2003 on because that is when Theo took over the Red Sox, but I think it is also important to check what Cashman did prior to that. I know the comparison can only begin in 2003, but for the sake of just analyzing Cashman, lets go back to 1998 - 2002...
I understand that a large part of those championship teams were developed prior to 1998, but Cashman acquired some significant players as well.
He was able to acquire Chuck Knoblauch almost immediately after taking over as GM, a trade that Bob Watson was unable to complete during his two years as GM.
He traded for Roger Clemens.
He didn't trade Andy Pettitte, even though there was pressure on him to do so.
He acquired David Wells (1997).
He signed El Duque.
He signed Mike Mussina.
Obviously we don't know that all of these moves were made because of him, but considering we're giving Epstein credit for the players the Red Sox acquired we might as well do the same for Cashman.