An SM58 is basically a 57 without the head - the head with the pop filter gives the 58 a boost in the midrange, that compliments most vocalists (depending on preference as well, of course).
I'd say the 58 would be a very good live vocal mic, maybe for some people suitable for studio recording purposes as well, but of course it would have a different effect compared to a good condenser mic.
As for the SM57... well, all I can say is that even those who spend tens and thousands of dollars on their studios and mics use it for their instrument recordings - from acoustic guitars and drums to guitar amps to anything else - it's one mic you won't regret buying if it applies to your recording needs.
