Some like the DFX, for them and their area and hunting needs, and can get good performance out of it.
On the other hand, I personally prefer the XLT, for ME and MY NEEDS, and have found that when I had both and worked them side-by-side in the field, and included three other White's models to get extra input, the DFX just didn't cut it. Yes, there were a couple of times I got slightly better dept in an audio response, but overall, the XLT's I used, with my three custom programs, gave smoother and cleaner hits on the same located targets, gave me a more accurate and better lock-on TID/VDI on most coins in the 4" plus range to the DFX, and so I have formed my own personal bias.
The DFX is an okay detector. I just prefer the XLT. Both have some adjustment functions that can mess up the performance if you get too carried away, so learn which ever you have, adjust it with care, then work it in the field with faith. I do, and so have several White's dealers and hunting friends I know who have used the XLT, gone to the DFX, and later returned to 'old faithful.'
Monte