There was an interesting post on this at Battersbox: Lee vs. Halladay. One thing they mentioned is that the batters that Lee has faced this year have a collective OPS of .732 while it is .766 (OPS = OBP + SLG).
But how should this difference affect each guy's ERA? I did not see it mentioned or discussed there (my apologies if it was). So I will take a look at this issue.
Based on data from all major league teams from 2001-2004, here is the relationship between OPS and runs per game
R/G = 13.26*OPS - 5.29
For all teams this year it is
R/G = 12.07*OPS - 4.39
If we multiply 13.26 times .034, the difference in the OPS of their opponents, we get .45. If we use 12.07, we get .41. If we add that to Lee's ERA of 2.36, we get 2.77 or 2.81. Halladay is at 2.77. That makes things pretty even.
But what if we look at DIPS ERA, an ERA computed based only on things the pitcher controls himself like strikeouts, walks and HRs (DIPS means defense independent and was developed by Voros McCracken). Lee has a 2.85 DIPS ERA and Halladay has 3.06. So then Lee would jump well above Halladay in ERA.
Now we don't how good the pitchers were that these batters faced. Maybe the batters who have a collective OPS of .732 (the ones Lee has faced) faced unusually good pitchers. Probably not, but we do need to note it. If not, then this analysis gives the edge to Halladay. Halladay came into today with an edge of 14 in IP (224 to 210). He pitched 7 more today. Given that he will end up with more IP (it might not be as much as 21, though depending on how much each guy pitches from now on), Halladay has a case for the Cy Young award.
Technical note: The standard error in the OPS/Runs regression was about .15 in each case or about 24 runs a season. Certainly not the best estimators around but decent and OPS is the stat at issue here.