The III has a more aggressive and inspiring Lead channel, but the R2 channel lacks gain for metal rhythm and doesn't balance well tonally with the other channels. The clean channel (R1) is severely compromised when everything is set to Kill for the Lead, but a nice compromise can be reached, especially in full power mode and with plenty of presence (even better if you have at least 1-2 single coils).
The IV has a stunning clean channel and Lead, but R2 again lacks gain for metal rhythm.
They really nailed it with the VII, truly a complete, balanced Mark amp, no pedals/mods needed. You can get Valdez tankers of gain on channel 2 and 3, and the totally isolated clean channel is C-L-E-A-N. The VII doesn't have the charismatic wail of the Quad though, or the anvil crushing chunk, although the anvil is expected when you have double the power (using the 2:90 ).

And the TriAxis / 2:90 is right there with the Quad for quality of tone and BIG BONES, although more complex to deal with.
I have all of the above amps, and play the Quad/2:90 and IV most.