according to the Shure website, the SM58 is for vocals, whilst the SM57 is more for instruments and general stuff.
although the Shure website also says "The Shure SM57 unidirectional dynamic microphone is exceptional for musical instrument pickup or for vocals."
SM58: 50-15000Hz
SM57: 40-15000Hz
they also have slightly different frequency response. (for what that is, to put it in simple terms, when u play guitar at high E string 24th fret, the volume seems to go down, whereas at bass E string 0 fret the volume sounds louder.)
i'll be getting a SM57 since it seems to be more versatile.
and ya, sorry for the mistake, it's a SM57 i'm looking out for.