Main difference between an SM and a PG is in the areas of ruggedness and handling noise. Metal parts are thinner or in some cases replaced by plastic. Heft a PG 58 and an SM 58 - the PG is lighter. Sound quality is good for the price, but I wouldn't expect a PG58 to survive 10 years of punk-rock shows without a major overhaul, as my first 3
SM58's have (replacing a
windscreen is not considered a major overhaul).
The main difference between a Beta 58 and an
SM58 is the magnet - the Beta uses neodymium versus the SM's Alnico V. A Neodymium magnet has a much stronger field than an Alnico V magnet of the same size and shape, so the Beta has a higher output for a given sound pressure, which lets you get by with less
gain in the signal path and, as a result, less noise. Well, my
system is pretty quiet with
SM58's. I don't see enough improvement out of a Beta to justify the price differential.
John