John Walsh has an excellent piece at Hardball Times, looking at the difference in OPS from year to year among players who move from the A.L. to the N.L., or vice versa.
120 players were in the sample, and an average, a player posted an OPS of 29 points greater while playing in the N.L. than in the A.L., which suggests that the pitching and defense is better in the A.L. than the N.L.
I'm just summarizing here...the results are interesting, but there are some questions on methodology I have (particularly the decision not to adjust for park effects) that has me concerned about the noise level, but it is a good piece, and definitely worth a read.