The DFX is a very complicated piece of equipment. When fully understood and properly set for your local conditions, it IS one of the top of the line machines available today. Sure there are a lot people raving about the Explorer and F-75, they too are top of the line machines. But they have their flaw also. Check the other forums, see how many people have had to send their F-75 to the factory for repairs and new coils "out of the box". When it works, it works great. The Explorer is a sophisticated top of the line machine, too. It's biggest draw back is the fact it to very noisey. If you can't stand the hear a constant chorus in your ears, then you just spent $1200 for a machine you will never use. Is the extra inch or two worth it? maybe. But start checking prices of "extra coils" and the price doubles. Again, you might like the noise, and and performance, but if you don't know what the machine is telling you, how will you know what a good target sounds like. Heck you might get the same depth with the DFX in mixed-mode audio as the Explorer, but most don't like all the noise that comes with that level of performance.
Don't rule out the DFX, just consider what you want, what you can learn and which machine feels and sounds the best through the headphones to you. I still think White's is going to come out with another top of the line machine this spring, so maybe waiting a couple of months would be best. Plus with the new release of the F-70, there maybe something there, let others "test them out" and wait to see if they hit the secondary market very fast. Plus White's and Minelab's seem to hold more value when it's time to change detectors. They may not be a concern to you, but I bet you get a better percentage of your investment back out of your XLT than you do out of the F4. -and I say this looking to trade a White's Eagle Spectrum for an F4. I have not had one, but want to try one out, and have a decent light weight detector to keep in the car for those emergency MD fixes.
Good luch with our ulitmate decision, but make sure you buy what you want, not what others tell you their machine is capable of.