Maybe it's just now shown on the SM58 spec sheet, but the frequency response at 1, 2 and 1/8th in. Suppose I'll order both and see which one works better.
Aaron,
The proximity effect for SM58 is similar to the Beta 58A, but Shure no longer posts those curves.
The Beta58A is hypercardioid, and has a bit less "working distance" than the SM58. It has more output due to the neo magnet structure, so requires a bit less gain from the board.
The Beta 57A windscreen is about 1/8" shorter than the Beta 58A, so one can get a bit more output from it.
Art