I've fought a lot with the few mics that I have: SM57, e609, and even my cheap MXL 990 (which I've been VERY happy with for vocal recording). If I could get a very well isolated room for the cab, I would use the 990... it really captures the sound of the cab, but it captures all of the wall reverberation and rattles from anything not bolted down so it's pretty useless to me right now. The e609 has been useless as well... I've heard it work well with some amps but I can't get anything but a highly scooped sound with very shrill high-end no matter where I place it on the speaker. The SM57 is what I've been relying on, but it sounds so 2-dimensional and doesn't capture any of the richness from the amp.
So I'm also looking to upgrade to either a MD421, SM7, or even try the e906 or i5.... but I also need a really nice preamp as well =P