If the Sovereign GT operates like the Sovereign XS, then it's a fine detector. I've never used the GT, but did use an XS for about 6 years.
You wouldn't go wrong with either unit. They will both get the job done with the exception of prospecting. The MXT is the clear choice there.
Both would work well for fresh water beach hunting. The GT would win on a Salt water beach though, especially at the waters edge. There are not many units that can compete with the Sovereign in a Salt water environment.
From my experience, I give a slight edge to the GT for coin/jewelry hunting. The biggest complaint I had with the Sovereign was its slow target recovery (finding another close-by target after signaling on the prior target), and its poor performance in an iron infested environment. Some of the ghost towns I hunt are literally paved with rusted iron. It's not that it won't hunt in those conditions, it's just way too slow. It will find the goodies, but it takes a lot of patience. I really liked not having to think about ground balance though.
I wasn't too happy with the MXT for coin/jewelry hunting. I'm sure it was operator error and not a fault of the machine. I just couldn't get it to perform in the typical coin/jewelry environment. I prefer other units for that venue. I mentioned some of the reasons in a post to Ralph below.
Relic hunting, my primary endeavor. The MXT gets the nod here. I've not found a better relic hunting unit. I'll give you an example. Years ago, after 3 years of research, I found a lonely outpost of Fort Wallace, Kansas. I had only seen a one sentence mention of it in an obscure journal, but never could find any info on it. Well, finally I got the information I needed to do a field search and found the site. This was a true virgin site, and the accoutrement's were laying on the surface. Myself, and several of my hunting partners at the time hunted this small fort for several years taking a couple thousand artifacts from this site. We hunted with all of the top-of-the-line units of the day. Since then I have gone back at least once a year over the last 15 years. As you can imagine, the finds have gotten very thin in the later years. Even after all that pressure, relics continue to surface from the same ground that has been truly pounded over and over. The last few years have given up mostly an occasional bullet, but mostly a rusted iron pants button or a few percussion caps. Last year I took the MXT out to this site for the first time. The signals were very far, and few between. I was hunting the same ground that I, and many others, have hunted tens of times. I was surprised at the number of percussion caps that were coming to surface. The MXT gives that high pitched squeal on non-ferrous that shouts "dig". I really like that mode, and it's the best I've used for relic hunting. Anyway, I got one of those high pitch squeals and started to dig. This was a deep target. To my surprise, out came a perfect company letter "B" from about 7 inches in a spot that I couldn't believe that any of us could have missed in the past. It just wasn't possible, I thought. That was probably the find that convinced me that the MXT is the finest relic hunting machine that I've used to date.
All in all, you really wouldn't go wrong with either unit. My personal summary is:
Relic Hunting = MXT
Coin/Jewelry Hunting = Sovereign
Beach Hunting = Sovereign
Prospecting = MXT
Again, those are my choices. You have to decide which is most important to you. I would try to give each a try before buying, if you can. Either would work well for you with the exception of Prospecting, where, in my opinion, the MXT is truly the choice.
Hope this is helpful.
OldeTymer
