I made the mistake of thinking I could save forty bucks and get by with the Shure PG-58.  I'm no vocalist but occasionally need to do announcements or accommodate a guest vocalist through an instrument amp (rather than a P.A.). Whatever the reason, the PG-58 simply doesn't have sufficient gain or oomph to be of much use with the Roland Cube / Peavey amps I try to employ it with (I use a 1/4" adapter with the XLR connector that comes with the mic).  When I borrow a Shure SM-58 mic from someone, the difference is instant and dramatic--it's a far more efficient (i.e. louder) mic, producing a full, clear sound, and without additional pre-amplification, demonstrating why it's the most popular microphone in the world.  For five bucks extra, I can't fathom a reason for avoiding the SM-58S, the version with an on-off switch.  It's a small price to pay for quick solutions to feedback or the unwanted pick-up of "private" comments by indiscreet band members.  And even if you're an instrumentalist, it may be wiser to select the SM-58 over the SM-57.  The former mic is a little more ostentatious, but it's also slightly more adaptable, and the screen can filter out a certain amount of wind noise and unwanted non-musical sounds. Above all, the SM-58 has proven itself to be the Sherman tank of microphones, simply unparalleled for ruggedness and durability.