The weight of the GPZ with a good harness and bungy is not a problem with the 14" coil, but I hear the 19" requires a steroid prescription for a full day in harness.
It's horses for courses though (bit of a pun there mentioning harness - not to give you the trotts), and operators rarely agree - some can't stand coming home with nothing to show, so the SDC's a winner there. Some can't stand coming home with 5 nuggets for 0.1g so the GPX and GPZ are winners there. On reefy or large deep gold, or in very hot grounds, the GPZ seems to have the advantage, but you're going to dig some serious holes. The SDC and GPZ machines are (IMO) more easily learned than the GPX, but I have had a blast with my GPX4500. It also depends greatly on the areas that you plan to prospect - very shallow, then SDC wins, deep and GPZ performs. Between the two is the trusty GPX with its myriad of coils depending on conditions.