This is just an educated guess, but by my responding to this, it will bump it up so that more people may see it, and hopefully someone who really knows will jump in. My understanding is that the main funtional difference between the 57 & 58 and their Beta versions is that while the regular versions are cardioid, the Betas are closer to a hypercardioid pattern. The off-axis rejection is much greater in the Betas, a highly desirable trait for LIVE work. I don't really see where this would be too advantageous for a home recording situation, especially micing a guitar cabinet, unless you were recording live, and had a combination of really lousy acoustics and no isolation. Just my guess, anyway. You didn't say what other gear you already have, or what you're trying to do exactly (is it just you, one track at a time, or are you recording your whole band at once, etc.). I personally would go for one plain vanilla 57, and apply the rest of the money to some other needed piece of gear (preamp, better 2nd mic, etc.), but you obviously have a better idea than I of what you're priorities are. What would you use the 2nd 57 for? Not that it wouldn't be a good idea to have another one - 57's are like money, you always use some more. But if , like many aound here, you're just recording yourself, one track at a time, then I would think that you would want a different 2nd mic, to add the possibility of more tonal variety. Just my opinion, of course. Sorry for rambling. I'm in one of those moods. (Cue Steve Martin playing "Ramblin' Guy)
Hope somebody answers your question. Good Luck.