From Shure's site:
The 57 and 58 microphones are actually based on the same cartridge design. The main difference is in the grille. The SM58 was designed for vocal applications, and therefore uses a ball grille with built in pop filter to eliminate plosives.
The SM57 is designed as an instrument microphone, where a smaller grille size is more practical and plosives are less of a concern. Subsequently, the SM57 does not use a ball grille with pop filtering and instead features an integral resonator/grille assembly, where the grille is actually part of the cartridge.
Each grille design places the diaphragm of each microphone in a different acoustical environment. The distance from the top of the grille to the diaphragm is shorter on the SM57 compared to that of the SM58, which allows for a more pronounced proximity effect through closer mic positioning. Additionally, the different resonator/grille assembly design of the SM57 produces a slightly higher output above 5 kHz.
It's All a Matter of Microphone Application
Although the SM57 was originally intended for instrument applications, there are plenty of examples through music history of it being used as a vocal microphone. This is because the increased proximity effect – and 5KHz frequency boost – can be used to the engineers advantage with certain vocalists. In other words, the same qualities that help instruments, can sometimes also bolster certain voices. It's all a matter of what works for your application. Experimentation, and using your ears remains the key factor.
So there you have it, they're actually pretty similar, with subtle, but potentially significant differences.
