Let me clarify things a bit here...
Yes both cartridges are identical (though different in the Beta models)
However, the different grilles DO change the frequency response. If you have ever A/B'd the mic's onstage, you will notice a clear 8k presence peak to the 58.
In addition, the shorter grill on the 57 allows tighter micing, resulting in:
1. Increased bass response (proximity effect)
2. Higher gain before feedback
3. Mo' output
Although, I prefer certain mics for certain applications, they are basically interchangable. No one will spank you if you use a 58 on the snare instead of a 57 ;-) Just use your ears and make sure it sounds good!