Owned a few 6000 Pro XLs, a QII, a few QXTs, and a few QXT Pros. Used an M6. My favorite is the QXT Pro. The QXT is the same exact machine except the Pro allows a couple clicks higher sensitivity and still is stable. Must be newer/less noise producing electronics in it.
The high frequency Whites units don't do well in my soil. A friend had a MXT but got rid of it for that reason. I'm a bigger fan of the older low frequency units. Seem to get better depth on silver and copper in my soil. Wish they'd come out with a new low frequency machine and some 12" or so DD coils for it to push the depths further.
Hunted around an XLT and a DFX but wasn't too impressed with what I saw. Not a big fan of all the controls on the flagship models. Waiting for Whites to come out with something low frequency again that is simple to use yet gets really good depth in my soil.
I thought the QXT/QXT Pro had just enough computer controls to do what you needed. Anything much more than that is overkill IMO. Only complaint about the QXT/QXT Pro was a lack of VDI numbers. Other than that I thought it was a fantastic blend of just the right amount of computer controls along with a quick VDI so there was no lag time.
The 6000 Pro XL was a good machine but it was slow and it really needed tone alerts so I didn't have to stare at the meter all the time when picking through trash.
That's what I liked about the QXT Pro. I didn't use any discrimination, not even for the iron or ground zones. Instead I'd assign high tones to the zones I was interested in and lows for the rest. With it's quick recovery, super fast VDI, and instant low/high tones it was killer working heavy trash looking for coins mixed in there. While it was great with the 9.5" stock coil in heavy trash, put a small 5 or 6" coil on that thing and it doesn't get any better in a fast machine (old or new) on the market today for working coins in heavy trash. It'll keep up with the best of the new machines that are fast under those conditions.
That said, recovery speed is over rated IMO. If you keep your sweep speed slow on a slow recovery machine there's no issue. It's just that there are days when you want to swing faster and can't. What's more important is how sharp the signal is that the coil is putting out. That's why I'm a big fan of DD coils these days, and in particular the even sharper field generated by the SEF 12x10. It's laser like in it's ability to slice between targets.
The QXT/QXT Pro was also the deepest machine I ever used for a while, but with the lack of bigger coils (and DD coils) to push the depths further I finally left. I will come back some day though if they make the kind of machine I talked about. Whites for sure has the best user friendly hardware in terms of weight and hardware design compared to some other machines.