What to Do, Oh what to do
Rebel73153, Hard Prospector has brought up a good point.
I have never used either of the two machines, I do own the GMT along with all the coils made for it by White's. The MXT and the GMT share some similar electronics when you are talking about the gold detecting section of the MXT and that ought to be a plus. The MXT does find gold but is it as good as the dedicated GMT? I've read that it is close but not quite as good.
When I decided that I wanted to look for gold with a detector I knew I wanted nothing but a machine built totally for being out in gold country looking for gold. I'd already tried my coin shooting machine in gold country and it was a no go. I'd owned the original Gold Bug and wanted nothing that was similar to it. I went to Whites as I've dealt with the company and have always been treated well by them and their machines have worked for me in the past.
I do not get out that often so I am not "skilled" in the use of my GMT however I've worked with it enough to be able to find gold with it in the same exact country I'd had the Gold Bug in and found nothing. Finding gold with a detector is an amazing experience. Using a gold detector in ways that augment other types of mining/prospecting so work is reduced and gold is insured is just a superb off shoot of a dedicated machine and that is what I've been able to do with my GMT. Took a little thinking out of the box but once I had achieved this ability I was totally sold. When you can confirm that a 5 gallon bucket of raw sample does not have a detectable quantity of gold in it then the labor on that bucket of dirt stops. That little trick can save a person at least an hour of additional hard work for nothing or very little if there are some really small fines in the sample that are not detectable.
I found this with my dedicated GMT using both the stock 6 x 10 & the 4 x 6 coils -

It is possible that a coin & gold detector could have found this as well but a dedicated machine certainly did. So the ~ 1/4 oz of gold nuggets is worth about $350 depending on spot prices and whether you can sell the nuggets for people to look at or melt down. $350 is about half the cost of the GMT.
Now, gold is Difficult to find! Don't think just because you've a dedicated machine that it is easy as it is not. The owner of the machine has to take on the responsibility to Learn how to use the machine and honestly I have not done a good job of becoming knowledgeable with mine. Likely that is why it took me 3 years to find gold with it. BUT, and here it comes, now I know enough about it to know that IF I put myself in an area where there is gold that this machine can and will find it. That is the other thing that the owner of a machine must do, put themselves in an area where there is gold.
So, learn the machine regardless of it being dedicated or multi-tasking And put yourself where you are standing on top of gold. Without these two in place, stick to detecting in the city for gold rings. If you like the looks of my gold on top of the U.S. fifty cent piece then follow the two Must Do's above.
The best of success to you on your choice of equipment, it is a difficult choice to make. Regards.......................63bkpkr
P.S. - also do not expect to make a living with a detector but you will see a lot of beautiful country while looking for gold. For me it is the adventure I'm after and the gold just makes it more adventuresome.

There is Gold near Roseburg Oregon