The SM57's been the stage & studio standard for mic'ing amps for decades, for good reason. It's worth stretching your budget a bit to get one instead of settling for a pretender, because you'll end up not only finding other good applications for it, but it will last 30 years or more, with at least minimal care.
Condensers are another thing, indeed musts, at least 1 each of a Large Diaphragm type for the higher SPL's and dynamic range of vocals and a Small Diaphragm type for acoustic guitars and instros and other low SPL/high freq range applications, for quality recordings. But they're not really the optimum choice to handle the higher SPL's from close-mic'ing guitar amps. They actually invite distortion, overdriven signals, and may not be focused enuff like a 57 can be. Unless you compress the crap out of them.
