I've been weighing the pros and cons between the DFX and the MXT, and originally was looking at the XLT also. What took that machine out of the contest for me was that the DFX has several improvements and increased capabilities for the little extra money involved. These include a faster "clock-speed" screen that is much quicker to register information on target hits, and probably the most important to me, the dual frequency capabilities. With the choice of correlate, best data, 3 kHz or 15 kHz, that gives you alot of possibilities that you don't have with the XLT. Other improvements incorporated into the DFX are user selectable ground filtering (from 2 to 6 filters) for a much wider range of ground conditions, adjustable sweep speed, bottlecap and hotrock rejection, and VDI normalization to sync the use of either or both frequencies to that of White's standard 6.6 kHz VID readings on earlier machines (including the XLT).
I think at this point, if I had intentions of using either machine for gold prospecting, I would go with the MXT. But since I tend to use specialized machines for that purpose when I do have the opportunity, nugget hunting with a multi-purpose machine is really not a concern. I personally see the advantage of having dual frequency or the choice of 3 or 15 kHz for beach or different inland environments of more benefit, not to mention full range notching capabilities, especially for hunting gold jewelry. While the MXT might be the more sensitive of the two in certain aspects due to design differences, I think the DFX is without doubt the more versatile of the two.
Ralph