It's been years since I have played a III, so consider that. I currently have a V which I just got back from Richard Lingenberg who is the top Mutron tech around. My take is there may be more sounds in the III, but the few that the V get really nail some great tones and are plenty for the few times one would use an ECF. I am want for nothing in that regard.
The other huge pluses for me are that they are cheaper, have a smaller footprint and they work on 9 volts, not the bulky odd power supply required by the III. From all I can tell, the Mutron products were exceptional, but any of them at this point are going to need some work. The one I have now has new capacitors, all the connections resoldered, the pot and switch cleaned, was re-calibrated, was changed to hard wire bypass with an LED and the old weird DC pin connector was changed out to a Boss style.
I have played a number of other ECFs and other than the III, there isn't anything that can touch this for the sound most guys here would need it for.