Each one is an improvement on the last, all be it minor differences. I went from an Explorer II SE Pro to a CTX and the latter has more adjustments for different sites and faster recovery, all the other differences are just add ons like GPS and a more informative screen, the latter is great for telling what is deep iron.
As said on other posts, learning what you have can far out way what machine you use.
There are 24 versions of a Spitfire (wiki) all with little modifications and tweaks, but at the end of the day they are all Spitfires
but I am sure budget permitting we would all prefer the latest one
Multifrequency is probably the most important difference above other makes and is what makes most Minelabs a good all round machine. For the record I also have a single frequency machine as the two types perform very differently.
Multifrequency for Pasture and Beach.
Single frequency for large air gaps, ie stubble and fluffy soil. Something Minlabes are not good at.