I kind of prefer the Audix I5 on heavy guitars myself. That said...the R121 is pretty darn good about 80% of the time.
I know the question was about mics, but I highly recommend Evidence Audio Lyric cable if you want 3D fatness...for both connecting from guitar to amp, and then for your microphone. It's amazing what you can get out of a guitar/amp if it's not being held back.
Guitar Cables, Instrument Cables and Speaker Cables from Evidence Audio
Brad