Hmmm, lets see.... the sm57 is a dynamic, cardioid microphone. It is very similar to the sm58, the main difference being the shape of grille. Bruce Bartlett said "I was a microphone engineer at Shure. The SM57 and SM58 are essentially the same, except that the SM57 was intended mainly for instruments and the SM58 for vocals. The SM58 has a ball grille that acts as a pop filter. The two mics have the same proximity effect at the same miking distance. Because of its smaller grille, the SM57 lets you get closer to it for more bass boost. But if you are 2 inches from the diaphragm in either mic, they have the same bass boost.They have a slightly different response at high frequencies because of the acoustical effects of the different grilles."
http://homerecording.com/bbs/archive/index.php/t-12391.htmlBehringer and Samson don't have a microphone with a grille similar to the sm57. But they do have one that looks like the sm58. These are the Samson q7 and the Behringer xm8500. But this is where the similarity to the sm58 ends. The materials, frequency response, proximity effect, and price are all different.
Most pros will choose the sm58 (or the sm57) over the other two pretenders because 1) it has been around for so long that they can readily predict how it will respond in a specific application, 2) it has proven to be VERY durable and dependable, and 3) it is one of the most affordable professional dynamic vocal mics. This is why most people here would recommend you opt for a real sm58 instead of taking your chances on something cheaper from Behringer or Samson, or even the bargain-basement Shure pg58. $100 is a bargain considering what you are getting. Provided you don't get a lemon, and with "proper" care, an sm58 will probably outlive all the rest of your audio devices. And if any of your children ever get involved in music or recording, they will be only too happy to inherit it from you. Now that's a good investment!
p.s. Other classic dynamic mics which I would be only too happy to inherit would be a Sennheiser 421 and an Electro-Voice re20. Now all I have to do is look for a rich audio fanatic with one of them, and get myself a fake birth certificate
Come to think of it, that would be a nice thread: "classic stuff I'd love to inherit".