SM58 has a removeable pop-filter/windscreen. Take it off and it's very similar to a SM57. Use SM57s for micing quitar amps, and SM58s for vocals. I've heard it said that the filter on the SM58 changes the high-end response. I think the filter makes the SM58 more robust than the SM57.
It's like comparin' as to what tastes better,Apples or Oranges?
They're both extremely worthwhile felxible mics whereas the 57 is excellent for micing amps and the 58 has good midrange for vocal applications.
Tom, the 57 has the presence peak -- the 58 has the peak also but it is less apparent (partly 'cos of the windscreen/foam.) Which is why, when you remove the top off of a '58, you essentially have a '57...