I've owned both. Both are great units. Which is better is more about what you want to do with them. What I like about the X-Terra is it is very light, breaks down real small for travel, and the functions/controls are easy to access. it is the better prospecting unit of the two.
The DFX offers more ability to tweak the unit, a plus or a minus depending on your preferences. What some people see as extra ability others see as complexity.
I really liked both but stuck with the DFX mainly because I have a Bigfoot coil that runs great on it. The DFX has more coil and probe accessories than the X-Terra.
I like my X-70 because of it's versatility and short learning curve. Also the big display numbers make it very easy on old eyes to read. I think...And here is where a person has to be honest with themselves...If a long learning curve, (On your behalf) is not your cup of tea; I would stay away from the DFX. Just my opinion, and you know what that's worth. . Good Luck