I have both an SM57 and a MXL 990. I use them both at the same time when recording electric guitar and I'm happy with the results. But if I only use one, its the SM57. The 990 just doesn't capture enough low or mid-low frequencies to give a beefy result when recorded on its own. Plus it cant handle the SPL of a cranked cabinet without distorting. I have to back it off farther than I'd like to (5' or 6' instead of the more traditional 3' or so). That reduces the low end it picks up as well, and adds a lot of room ambience to the end result. The more ambience, the more distant it sounds in a mix. The more distant, the thinner it sounds. MXL makes a version of the 990 that has a -10dB pad switch as well as a low-end rolloff switch. That might help towards making it a better mic for loud amps.
Definitely invest in a SM57. It has so many uses, and it'll last you forever. It's a very worthwhile investment for a home studio.
And don't tell Sonixx I said this, but the first thing I'd try to remedy thin-sounding guitars is to lower the amount of gain/distortion that you're using. Too much distortion tends to make an electric guitar part sound fizzy and thin in a mix. Remember that how an amp sounds to your ears in a room is usually going to be different to how it sounds to a microphone that's right on the amp's grille cloth. Listen to the amp mic via headphones and hear how the sound changes as you move the mic around (or better yet, let your friend move the mic around slowly and you can play and listen for audible changes due to mic positioning).