I have both although my wife took over the DFX when I got the E-trac. I can tell you that they are very different machines. The DFX seems, at first, to be the easier machine to use. After all, it gives you tones, an icon, a number, and a graph. You would think you could nail what is down there every time. The E-trac gives you tones, TWO numbers (ferrous and conductive) and a graph (that seems mostly useless to me). Of course, both detectors have a zillion possible adjustments and possible programs. Coming from the DFX, when I first got the E-trac I got really frustrated with it. Frustrated enough that I went back to the DFX for a while. The icon and graph seemed so much more familiar and reassuring. Then it dawned on me that I was digging just as much trash with the DFX as I had with the E-trac. It was time to give the E-trac another chance. Others had told me that the E-trac takes about 30 hours to learn and that one day I'd just "get it". I have to admit that they were correct. I don't know how to explain it but one day I got home with a fistfull of coins and only three pull tabs. More importantly, I had several nickles but only those three pull tabs. In my experience nickles and pull tabs go together and that's just a fact of life. Until now, apparently. I must have "gotten it".
As for relative depth capability I can only say that I took the E-trac to an old church yard that I knew I had hunted pretty thoroughly. As a matter fact I had searched it with an old Garrett Grand Master Hunter, again with a Garrett Ace 250, then with the DFX before I showed up with the E-trac and the standard 11" coil. I didn't expect much. I was surprised when I started pulling coins from from the ground that I must have walked right over with another machine and probably more than one.
If you don't already have one, you can greatly speed the E-trac learning process by making a target garden. I have different coins, pull tabs, and bits of trash buried at different depths. I know what's down there at each spot so I can scan it while listening to the tones and seeing the numbers. It helped me a great deal. Forget air testing the E-trac.
One other major difference between the DFX and E-trac is sweep speed. Get ready to slow down. A lot. If the E-trac has a shortcoming it's that the microprocessor is sorting a huge amount of data and I'm guessing it is right at the edge of its capability. I have found that I get the best results when a sweep from one side to the other takes about 4 seconds. That seems glacially slow compared to my sweep speed with the DFX and any other detector I've used in the past. Take a deep breath, relax, and let the coil just float along over the surface.
It's a great machine!
Storm