SM-58s, by Shure, are world renowned as some of the best live mics out there. A perfect balance of price, sound quality, and durability. Partnered with the SM-57, they are definitely the most popular mics for on stage. So what is the difference between the '57 and '58? To be honest not much, they are the same price, sensitivity and capsule. The '57 is mostly used as an instrument mic, the '58 is simply a '57 with a ball foam filter on it.
So there is an awesome trick to do, let's say you don't have enough '57s but a spare '58. This has happened to me when setting up for a gig. Simply unscrew the ball top and it becomes an SM-57. Since the ball filter is the only thing that really makes the difference to the sound of the two mics, a topless '58 acts like it's instrument counterpart.
So this sounds like it's useful in a limited scenario but you can use it to your advantage. If you want a selection of mics for your stage, let's say five SM-57s and five SM-58s, you'd be just as well getting ten SM-58s. Then you are able to have any ratio of each mic depending on the event.
Likewise,if you are on a limited budget and want an instrument mic and a vocal mic, just buy a couple of SM-58s and take the ball tops off when needed for an instrument. This way you have the flexibility to record two vocals/instruments at once.