Do you want a waterproof machine for water hunting or to use in the rain? Salt or Fresh? I believe the MXT is sealed to allow you to use it in moderate rain conditons. If you are looking to take a dip for prospectiing or freshwater target hunting purposes with the MX Sport (I own one but have not extensively water hunted with it) then one knock against it is that the stock coil and OEM coils Whites put out for it are floaters and make it hard to hold the coil under water (less of a problem for the stock DD than for the solid covered accessory coils that White's markets exclusively for the MX Sport). I think that you can now get third-party open coils and that may may alleviate this issue. I will say this, I own both the MX Sport and MXT and they are really two different machines despite some of their similarities, I do not consider the MX Sport a waterproof MXT. I primarily relic hunt, but personally prefer the MXT to the MX Sport though neither is my primary relic machine (that's a Minelab GPX backed up by a Deus because I am relegated to highly mineralized soil in my local region) because the analog controls are easier to deal with in the field for varying conditions than flipping through a lot of menus. I do like the Sport's big display and backlight, but have not pulled a lot of keepers with it. But to be fair, I don't have the swing hours either. I got it as an alternative to the AT Pro (for beach hunting) and like it a lot better than the AT Pro, but I have since been able to effectively waterproof the Deus and have primarily stuck with that when I am going to get wet. As far as prospecting is concerned the 13.9 khz operating frequency is a good "all around" general purpose frequency, but this thing is not a nugget hunter and is basically the same as the MXT's 14 khz as far as gold is concerned.
I also have experience with the TDI. Again, I ended up going with the GPX because it is deeper and where I hunt things have been pretty picked over, but the TDI is a great relic machine in it's own right and a lot less expensive. Don't know about much about the GMT. I think if you went with the MXT, MX Sport, and TDI you would have some great mid-to-high end machines to cover the bases on just about any type of hunting other than wet/surf salt beach (MX Sport is OK on dry sand and will "work" in wet sand and surf, but you would be better served by a waterproof PI machine or multifrequency VLF machine suited for salt beach purposes IMO).
HTH