The Beta 57 and 58, Sm7, SM58, and SM57 all use the same old Unidyne III design type cartridge and all have the same "voice". This has been discussed, tested, graphed, blindfold shootouts, etc. and it is extremely hard if not almost impossible to distinguish a difference in the voice. If you can "learn" a difference from consistent samples a simple move or pop filter will cause one to take the others characteristic or vice versa. All differences in sound are caused by the distance the capsule is mounted from the end and slight output impedance differences, patterns of sensitivity from physical design, and slight output impedance differences from the transformer used. Again, the voice is the same.
There was a recent shootout in which experienced industry professionals could not identify consistently which mic was an SM7 and which a SM57 in a vocal recording. Dispite the physical differences the cartridge is the same basic design in all and maintains the same "voice". The console or mixer you plug into or distance, angle, etc, can easily cause physical characteristic to overlap in performance. You would not be able to identify which mic is which in a recording if used in random applications (varied distances, pre-amps, etc.) They all have the same "voice" and can yeild near identical results.
Here is the response patterns from a voice done by 3D Audio's Lynn Fuston. Shows SM7 and SM57 patterns overlapped. The results are so close it's actually shocking. You would get much more variation in comparing a pair of U87's.
Fuston points out that the key is simply the distance you place your mouth from the diaphragm itself. With lips touching the grill the 57 gains lows over the SM7 because of closer distance to the diaphram due to design. Back the mic away 1/4" to compensate for distance and low response evens out. The fundamental voice and mid frequencies do not really change.
With pop filter off and diaphragms at equal distance the SM7 and SM57 will yeild pretty much identical results.
Don't have an SM7 and need a good vocal recording? Pop on a foam filter and no one will know but you.
Don't have a 57 but you do have a 58 and need a guitar mic? Just move it a 1/4" closer than you would a 57 and you have a match...or just unscrew the pop filter. Same with a Beta 57 or 58 and compensate for output variations at the board (if it makes you feel better). There's no extra magic from the number designations, they all have the old Unidyne III type design cartridge and they all have the same voice.