You want to calculate park factors? Beckett's still clearly better with a 133 ERA+ compared to Sabathia's 123. I'm sure you don't have a desire to debate this because the numbers clearly show Beckett's been better, and the more you examine them, the better Beckett looks. The only thing that is close between their two is their non-park adjusted, non-defense adjusted ERA. Sabathia's been extremely lucky in that he's pitched to a mid .200's BABIP and he's had a lot of defensive help. His ERA will likely start to climb as those things even out.
With Beckett's it's been the opposite. He's been unlucky on balls put into play and he's had very poor fielding behind him. If you adjust for those factors, his FIP shows that he'd have an ERA in the low 3's.