First off, I don't know what a MXT E-track or DFX E-track is. I know Minelab makes an E-Trac, but I don't know if Whites makes an E-Track.
The difference between the MXT and MXT 300 is that the 300 has a bigger coil ( about 12 inches) instead of the older stock coil of 9.5 inches. The difference between the DFX and DFX 300 is the same thing.
There is a difference between an MXT and DFX, one of the main differences is the DFX is a dual frequency machine, and will work much better on salt water beaches. The MXT is simpler to use and is a great relic, coin and even gold prospecting machine. The DFX is much more programmable and has lots of options you can change/switch. Much more customizable in terms of setting parameters and creating programs for different sites and environments.
You can read up on the differences on the Whites Electronics site.
I never used a Minelab E-Trac so I cannot comment on it but I hear and read its a great detector too.