Maybe I can put some spin on this.
What is it that makes one detector better than another ?
Most people would say depth, then hesitate & mention recovery speed, think a little longer & weight will creep into the equation & so on. So what I'm trying to get across is that we would all like to 'pigeonhole' detectors as mentioned above but in reality the differences between detectors is very subtle, what I have noticed over the years working with some excellent engineers, excellent designers & also being at the sharp end of sales there has been progress over the years but not in leaps & bounds or has there ? 1999 we were offered a new multi-frequency detector that seemed at the time to be deeper than equivalent machines on the market, 2003 saw a new kid on the block, whilst not as deep the general consensus was that seemed to find more so our perception of detectors was starting to change, depth was only one part of what made a good detector.
What I see now is detectors becoming more refined in many aspects, wireless headphones were something we'd always needed/wanted, wireless coil has so many advantages we didn't know we needed till we got them, these advances make for a lighter, faster more efficient detector that we are seeing today & it's this word 'efficient' that gives us more finds in our pouch, modern pinpoint probes have added to the equation.
So to answer Keith's opening question, it's not always a case of pinpointing the exact reason some are better than others, if you could then manufacturers would sit up & take notice more often to threads like this, they may know the theories & marketing reasons but again we are at the sharp end, we hear what customers say, we take onboard what comments are made, sure you can dismiss some of the comments but we also get a good feel for what 'detectorists' need, like & more importantly dislike.