I own both the Metal Zone and the Little Big Muff... and these are two totally different beasts.
The Muff gets a great big, thick, smooth fuzz. Fantastic low-end. Usually scoops your mids a bit.
Metal Zone is completely different. I use mine for a more subtle distortion that I can't achieve with the Muff. I have the bass maxed out, and I still use a little low end... but not too much. (I use a Pbass, btw. If I used, say, an MM or something that's known for a bigger low end, I'd probably lose more lows) The Metal Zone is good, to me, because I can boost the low mids... which are quite boosted. This creates kind of a weird solo tone, but in a band setting, I cut through better than a muff. It also allows the nuances of your bass and technique to come through more than a muff. Although I generally like many things about the MZ-2, I sometimes feel that I'm "settling" and that I can find another pedal with all of the good qualities... without the bad. I'm drooling over the Crowther Hotcakes (XLF version) right now. I'm hoping that it'll provide a nice medium-heavy distortion that can cut through the mix while preserving my tone... without sounding thin, like the Metal Zone does without the SABDDI.
I also want a Zvex Woolly Mammoth (outrageously expensive) for huge, mean fuzz that can be gated for cool synth sounds (yay!) and an Ezekiel by D*A*M for everything from the perfect OD to the perfect fuzz.