I have used the SM58, the BETA58A and the SM7B on a variety of studio vocal recordings. I like all three for different applications. With microphones, I have a hard time telling someone which one is better for his or her voice because I haven't heard the voice yet. That is why I buy as many mics as I can so I have them available when singers walk into the studio. I have tracked vocals on a Shure RS20 (RS = RadioShack), and while the RS line of Shure mics aren't the greatest sounding, that rcording need that sound with that singer. So really, which ever mic you buy, you will benefit greatly from having the tools you need. There isn't one screwdriver for every screw in the world. There isn't one mic for every voice.
I still did not close it for myself and found another possible solution - Audix OM5 and OM7. Reviews on Harmony Central are very positive and people place these mikes above Shure SM58 and Beta. Is anybody here who knows them?