i have both a 609 and 906, i like them both, i use them for things where i wouldn't really want a 57 quality, both are softer sounding than the 57, i tend to mix them with other mics, say for incredibly bright leads, ill mix in either mic to add smoother, rounder quality to it.
i probably wouldn't use these mics for any mids heavy material, like a good rhythm track, just a tad on the dark side.
but they a good for more classic tones, where treble and presence are boosted and the mics can soften the high end a bit.
but for what i like, the 421 has the qualities you want in a multi mic setup, its good lowend response for guitar, good mid response, and usable high end, it just fills in and expands on what the 57 does well. makes it sound huge when done correctly.
but if you want a good round smooth quality to your guitars, then get either a 609 or 906, i tend to use the 906 more.