I've read that if you remove the pop-shield from an SM58 you've essentially got an SM57 so the SM58 is the one I'll go for. When I learn how to sing better, and work out what I don't like/don't like about the '58 I'll go mic shopping again :)
You're actually pretty much on the ball with that one; both microphones contain identical capsules.
It's also worth bearing in mind though, that neither sound good. I've had a lot of experience with microphones over the last couple of years, and the 57s+58s, while they are great for cutting through live, sound horrible for guitar/vox on their own. They are very loud, harsh, and often shrill microphones. Good results can be achieved using them in conjunction with other mics (or with very good positioning), as often they add the right 'kick' to a sound, but generally the only thing i use either of them for is a 57 below the snare.
