What are you looking for in a mic? If you're looking for a good condenser, I like the Oktava MK219. It's not overly bright and that's helpful on electric guitars. If you want something completely different, try a Shure Green Bullet. If you haven't tried an i5 yet, it's got some similarities to the 57, but it's still a very different microphone and worth checking out. The Sennheiser 421 and Shure Sm7 are two more commonly used dynamics. There are also a few other cheap ribbons besides the Fathead that work pretty well.
You can use just about any mic on a guitar amp. I've even used SDC's with pleasing results. I'm just not a fan of most of MXL's mics. I have a few that are supposed to be some of their better models, and while their usable, they don't really blow me away.
I guess my money would be better served buying a nice condenser for recording vocals. I love my Fathead - absolutely the best guitar mic I've ever used and I've used a bunch. Musicians Friend's stupid deal of the day was for the MXL ($79), so I got curious.