All other things being equal, for EVERYONE 9mm is easier than .40 when you're talking about multiple shot strings... its physics... don't know how else to say it guys. Some people and some platforms may make the difference negligible, but it must be there.
The idea that you "shoot X caliber more accurately than X" (obviously talking about defensive accuracy here, not Camp Perry) is either in your head or a factor of the platform (eg- comparing accuracy from a tuned 1911 .45 with a good trigger and a H&K DOA USP in 9mm is apples and oranges...). It is also possibly missing the point that we are talking about multiple shots, so accuracy is only one factor, recoil management is what I am focusing on (given that .38, .357, 9mm, .40, .45, 10mm and .44 are all capable of being fired accurately in a self defense situation) ... based on the empirical evidence.
With that, I'm going to bow out of this one and probably lock the thread down in a day or so if no new info is added.
Good discussion and, I think VERY THOROUGH... we're close to a "'nuff said" on all fronts.
-RJP