Here's the "subject matter expert"! (LOL)
IMHO, for MOST vocalists the Beta 58 is superior to the 'ol SM58.
If I was getting a Beta 58 or Beta 57, however, a nice used one
off e-bay is a better way to go than one new.
Keep in mind that many pro engineers feel the SM57 along with a pop screen (not foam!) is superior to a SM58 for recording.
In fact the supposedly "better" Beta 57 runs a dead heat with my
SM57 on my voice, with each having a signature sound.
On the other hand, if I were singing the lead in a Stones tribute
band and looking for that "sound", it would be a SM58 in a jump'in jack flash!
One of the advantages of the Beta's is that they run hotter than a "regular" 57 or 58, thereby needing less gain from our often less than stellar mixers/mic pre's.
Chris
P.S. On my voice it took a lot of time to finally figure out the
correct placement of a Beta 57 due to mild sibilance issues
as it's brighter than a SM57.
A Shure SM57 is easier for me to set up quickly due to its
relative lack of top end compared to the Beta.