buehrle's a better pitcher in the season, It's yet to be seen if MB is a real "Gamer" like Pettitte is. Pettitte has a 14-4 post season record, and a 4-1 WS record (even though his Post season ERA is markedly higher than his career ERA). Until MB really shuts down good teams in BIG games he's going to make less money.
Pettite had the added advantage of having the Yankees to bid agaisnt.. That significantly raises his market value.