Shure Beta 57A vs Shure Beta 58A
Both are supercardioid dynamics with a neodymium magnet. As with the SM57 and SM58, the difference is what each one is designed for.
Which one to choose
Being supercardioid, they give more output and isolate each source better than the equivalent SM models, at the cost of picking up a little from behind: live, wedges go at an angle rather than directly behind the mic. The Beta 58A has a spherical grille with a built-in pop filter, for singing; the Beta 57A has a compact, tough grille for close-miking snare, toms and amps. For vocals, the Beta 58A; for instruments, the Beta 57A.