Gamma and MXT are both good machines, but very different. To say one is "better" than the other would miss the point. The right question to ask is which one is a better fit to your personal preferences.
I designed 'em both.
You might want also to consider the Omega, the F5, the GBP/G2 (Fisher/Teknetics) and the F70, which are other good performing machines in the same general price range of the MXT or a little lower, it's been a while since I looked at price tags. I designed all of those as well.
Of the machines I've named, the Gamma is the least expensive. Its basic performance is about the same as the Omega, but between those two I much prefer the Omega user interface. Also, most folks would probably say that if you get either an Omega or a Gamma, get an 11 inch searchcoil for it. I believe the Omega can be ordered with the 11 inch searchcoil but don't believe that's the case with the Gamma.
--Dave J.