The SM-57's been considered a penultimate guitar amp mic for years but actually is a more versatile all-purpose mic than the 58 b/c of the 58's proximity effect.
Skillful EQ'ing and injecting it through a tube pre-amp makes either mic excellent for vocals, but the 58's better for beefing up thin, reedy or high-pitched voices.
The 58's classic strength is close-mic'd stage vocals, limiting intruding background sounds fairly effectively.
Start with the 57, learn how to process it effectively in different applications and you may/may not find you need another mic, but if you do, get a 58.
Once you've got these 2 dynamics in your mic locker, for recording in a controlled sound environment you can look at adding a Large Diaphragm Condenser for vocals and Small Diaprhagm Condenser or 2 for acoustic instros or high freq percussion. Good condensers can be had in the same price range as the 57/58.