Kikling;
If you want to have one mic double for both chores, you'd be better off buying the 58. It's a lot easier to simply take the head off the 58 to make it into a 57 than to jury rig something for a 57 in order to make it a 58. Besides, the 58 can sound just fine for instruments with the head on. In fact, it's my favorite electric guitar mic like that.