I always use a condenser mic for vocals when possible .. but the original question was SM57 or SM58 (?)
You made the point....as I said earlier......
NEITHER the 57 nor the 58.....
Go buy a medium priced condenser mic.
But as an aside to answer the question: I all depends on the application. I hear 57's are great for drums and cabinets.... 58's seem to be sweet for vocals.... but I don't like using one for vocals in a recording studio situation. I have a 58 and used it out of necessity, only up to the day I got the Rode condenser. The Condenser mics are the best bet there. Realize it will pick up everything.
I was recording a singer in my studio and my wife was working downstairs and was working on some of that poster board paper stuff. I could hear it when the paper crinkled as she laid it out on the floor, in the vocalist's track later on playback. Funny thing was we didn't hear it when it happened. But there it was, on the playback. It picks up dogs barking, trucks and cars, planes, even the AC coming on and off..... I have learned to wok around those noises that it picks up.