I like using a combination of mics on my amps. I've used up to 8 mics simultaneously on one speaker. I use any combination or an SM57, a Senn e609s,
a Beyer M201tg, an MD421 and a AKG D112 on the grill and I've used all kinds of SDC's and LDC's at distance. I find I usually like having at least one average dynamic (the sm57 and e609), a Large Dynamic (the MD421 and D112) and an LDC at distance (usually my 4040 or NTk). I would really love to pick up a ribbon mic and try that instead of the LDC, but I've got other budgeting issues right now.
I would say you should look at picking up a complementing mic to the SM57. Something quite different. The Condensors you've linked from MXL are a good start, but you might want to think beyond just the guitar end of things. What other mics do you have? What kinds of pres are you using? What kind of results do you want? What else are you recording and how are you recording it?
If all you have is the 57, I would say you should pick up a decent LDC next. If your budget is 300 I would say that an
AT4040 would be a good mic choice - it's clean and flat and sounds good on most things (IMO). After that I would get a pair of SDC's, matched if you can, to do stereo recording like acoustic guitar and overheads, and then get a Large Dynamic like the D112. With those five types of mics (1 dynamic, 1 LDC, 2 SDC's, i Large Dynamic) you can record everything you need in a band well - Drums, Guitar, Bass, Vocals...you might want to add a good DI for Bass and Keys but other than that, an average Home studio can get away with just 5 mics...
Jacob