Hey!
Although the SM58 is best known as a vocal mic, I've gotten some very nice results from using it to mic up an acoustic guitar. Haven't worked that much with the beta87 though. I'd say, experiment with both while you can.
It ultimately depends on where you're using the mic. i.e. male/female singer, genre etc.
Personally, I'd pick the 58.

Eoin

They are both good mics and it is totally a matter of opinion. On instruments, I prefer the 87, it is more transparent and detailed. As far as vocals are concerned the 58 has more of a tone to it while the 87 is again, more transparent. Which one you use totally depends on the voice. I don't have the strongest voice, so for me the 87 is too revealing. The 58 works better with my voice. Which one works best for you is 100% your opinion.

Wes

Thsanks for the replies guys, you're confirming what I've been thinking for the past while as well.
The 87 worked very well on female vox, nice airy highs, but my voice felt very exposed on it. I don't have the best voice either, and the 58's warmth did alot for me.

The winner is..SM58 8)