Last I checked, Rodriguez leads the league in game winning RBIs. Not to mention, with Sheffield, and Matsui getting hurt throughout the year (yes playing through it), Rodriguez has started every game this year. He's batted from 2nd to 5th in the order, something Ortiz hasn't done; making him even more valuable because you can actually move him around for the benefit of other players. Ortiz hits in a hitter's ballpark, but Rodriguez is still a better hitter at Yankee Stadium than Ortiz is at Fenway. He's also arguably just as good on the road as Ortiz with the exception of the HRs. As far as playing in a tight race down the stretch, the numbers in August and September combined as of last night are:
David Ortiz: 21 HRs, 52 RBIs .311/.415/.737
Alex Rodriguez: 17 HRs, 40 RBIs .313/.407/.636
You also have to keep in mind that Alex has moved around in the lineup. Batting 2nd, he won't recieve as many RBI opportunities as Ortiz. The slugging percentage in my mind, is made up by Alex playing well at third base.