First, let my state, I am no expert. I'm a user of the original Sovereign and the MXT. I have many hours of hunting with both. I used the DFX for several years and feel like I learned it real well. All the settings on the DFX were constantly needing to be changed to mate with the site and types of targets I was after. I found a great deal of money and jewelry with it.
Finally, I let it go to a friend and bought the MXT because it was better for prospecting and the fact that coils I owned would interchange. I immediately started pulling coins from areas that I had hunted repeatably with the DFX, especially nickles. I hunt mostly in Relic mode and go by the confidence bar and tone, not the icons. One thing that bothers me with the Whites is this: Several times I have hunted and chose to dig all signals. I got a VDI reading of 40 something or any number that would not lead me to think it was a coin. Upon digging, it would be a dime, nickle, or a coin that the VDI should have been reading much higher. Re sweep the hole and register another target. What I actually have found is a coin spill that is 10-20 coins and the Whites acts like it is averaging or doing something screwy, not able to distinguish a solid coin VDI. This has happened on several hunts.
After reading many good things on this site about the Sovereign I placed a bib for one and won. Glad I did! Clive ? has several books I studied and after using the SOV a while I began to hear the tones. It ain't easy. I'm pushing 60 and all the years of loud banging at my job has worked on my ears. I have went back over my old sites that were hunted with the WHITES machines and found more coins, deeper. The original Coinsearch coil rang up three dimes in a play area that were over 10 inches deep, good clear signals. I had swept this area repeatably with both previous machines, neither even hinted the dimes were there. On recoveries with multiple coins in the same hole, the Sov and it's tones identifies them as good targets every time. The 15 inch wot and the Sunray intruder 12 round out the package. The Sov so impressed me that I bought a second one for a back up. Some on this site have stated that the GT is the best Sov yet. I can't imaging it being any better than what I own!!
So, do I like my MXT? Yes! With the Bigfoot coil it's hard to beat on a large field with lots of 3-4 inch deep targets. With the 4x6 DD, trashy sites are a pleasure to work. I have the 12 inch coil as well as the 9.5 but they rarely see use.
I hope I haven't sounded like a rep for Minelab, just sharing my experience with each brand. Luckily, none of my units have needed repair so I have no personal knowledge in that area. But, I have read good things on this site about both companies. I know this is the MXT forum and please don't think I am bashing a Whites. I cut my teeth on them and still use one, I'll always have it. Do keep an open mind though and use what works best in your environment.
I hope this helps in your choice for the detector that is right for you.