I keep reading that they are the same mic but with different windscreens. Take the ball off the 58 and its supposed to be just like a 57. Be careful with it though.
...they both use the same cartridge--i.e. the same freq. response--but, the ball grill on the 58 creates a slight low-end roll-off--which isn't a problem for a vocalist because no one can sing in that freq. range...they are considered "dynamic" because they're tuned to "brighten" or accentuate mids.
Another little known fact about the SM57 & SM58 is that they have a natural presence peak about 6-8khz because they were initially designed to work with the Shure "Vocal Master" system which had no high frequency drivers.