They are designed different, for example, the MXT has Jimmy Sierra Normandi's (famous gold person) specs for prospecting in it. The display is often different, with Iron and Ground Phase playing a big part. Finally, gold detectors usually work at a higher kHz than regular detectors. The MXT works at 14 kHz, the Minelab 705 Gold works at 18.75 kHz and the Gold Bug works at 19 kHz. White's GMT and GMZ work in the 50 kHz range!!!
Dusty
You also need to factor in coil size. Most prospecting detectors are built with coils on the smaller side in order to detect smaller objects (aka gold flakes).
Dusty just reminded me, also look for DD coil configurations in prospecting detectors to deal with the ground mineralization typically encountered when prospecting.Yup, my Gold Bug SE came with the 5" DD coil....works great but takes forever to cover a large area.....
Dusty
So, Minelab's marketing promotes FSB:
FBS is Minelab’s patented multiple frequency metal detection technology. FBS transmits a broad range of 28 frequencies from 1.5 kHz to 100 kHz. The increased frequency range allows FBS technology to offer more accurate target identification, improved depth of detection and improved sensitivity. FBS technology can be found in Minelab’s E-TRAC and Safari metal detectors.
Wouldn't that make it a good candidate for a gold detector. Or is having too many frequencies, too much and too broad, for hunting specific gold? You mention Iron and Ground Phase, perhaps these models are lacking in that dept, unlike a gold-specific model.