Okay, after 2 minutes on the Shure website, I've found the basic differences. The 57 and 58 are cardioid - uni directional, whereas
Beta 57 and 58 are hypercardioid - very unidirectional.
The SM57's and 58's are standard dynamic mikes, whereas the Beta mikes use a neodymium magnet. From what I've read about these, they give the mike a more condenser like frequency response, and as the site says, a higher output - which helps with your s/n if your mixer's good enough. And there's a $100 price difference.
Personally I would go with
the SM57 and 58's, not because they're cheaper, but they're the industry standard, and they're also part of that ideal sound in are heads, as they've been used in so many recordings, but obviously it's up to you. If you want better sounding vocals, why don't you invest your money in a condenser?