Don't quote me, but I think the 57 has a little tighter cardioid pattern, which is good for minimalizing some of the hi-hat bleed you will inevitably get.
I've heard it said in certain circles that they are the same mic if one unscrews the ball on the 58...like the ball is responsible for the slight change in the graph between them.
They are essentially the same thing. I use my 58 w/o the ball for guitar all the time. On the other hand, I would really hate for you to take the ball off of your 58 and hit it with a drumstick.