My personal choice excluding the DFX, is the M-6. If you intend to go nugget hunting, the MXT.
M6 represents the latest model, using the MXT's digital chip as a foundation, and the engineers re-worked the software resulting in
* Quieter Operation
* Seven Tone ID System (Two Tones in the Ferrous Zone Alone).
* Visual ID exhibits better lock on characteristics on deeper targets.
Regarding the Quantum XT.... until I got my hands on one of the early MXT prototypes, the Quantum XT was my Favorite Turf Detector with exceptional Discrimination Depth.
The XL-PRO was the last of an outstanding Line Up, especially after White's introduced the Liberty DI Signal Balance Control in place of the Preset Gain Controls. The added TR Discrimination complimenting the Motion Discrimination Circuit enhances the overall performance of this Analog Detector... Both Are Keepers, even though they have recently been discontinued, their individual language is very unique to each detector.
By the Way, I still have several of each units in stock. Email me if interested.