A while ago I actually called Shure to find out what the difference is between a 57 & a 58. They told me that the electronics are identical but the shape of the windscreen and body are taylored towards the application. 57 body for clips, 58 for hands, 57 head for instrument (or other non popping/proximity source) 58 has more windscreen for handling wind noise and the PP factor, as well as ESSSSS. So 57 stands, 58 hands.
Here are some 57's at work with Phatty screens!
Here's a 58 bieng totally abused!
Bri