Yesterday, The New York Times had an article by Alan Schwarz that quantified the the average improvement AL starting pitchers experienced when they moved to the NL and the average deprovement NL pitchers experienced when they moved to the AL. From 2000 through 2005, there were 29 AL emigrants and 28 NL ones:
Mr Schwarz also applies ERA+ to make the case even stronger. (Afterall, there is a certain cache to making cases more complicated by appling even less intuitive statistics!) Using this stat:
Whatever method fantasy leaguers use, there is statistical support for the common sense adage that AL pitchers benefit from changing leagues while NL ones who change leagues do not. I do wonder what the pre-PED/post-PED splits are, though.