are going to be the only one who will know exactly what it is you're looking for in a detector, and what will 'fit' with your skill and patience level.
The "unknowns" are these:
What do YOU plan to hunt for? Coins? Relics?
Where will you be hunting? Low-mineralized ground? Highly mineralized ground? Consistent terrain? Rough, rock-strewn terrain? Trashy sites? Clean sites?
What makes and models do presently use, and which ones have you had before? What features did/do you like about them? What features did/do you wish you had.
I've had my share of Minelab Sovereigns and Explorers. I learned them and I understand them, and I have a solid appreciation for what they can do. I also recognize their weaknesses, for me and what I want, so it makes it easier for ME to figure out an answer to your vague question as it relates to ME and MY desires. Thus, I'll answer it from MY viewpoint.
► Which is better for the money? Well you can pick up some older used Sovereigns at a reasonable price, but you did ask about the Sov. GT and X-Terra, of which there are two models. The turn-on-and-go X-Terra 30 and the somewhat more adjustable and versatile X-Terra 50.
The X-Terra 30 has a 3-tone audio system and the X-Terra 50 has a 4-tone audio system. The Sovereign GT will provide you with more audio tones and this is something that some people like while others don't care for it.
The two X-Terra's come with a visual target ID display, while a meter for the Sov. GT will cost you extra.
The Sovereign will have to be swept at a much slower sweep speed than the X-Terra in order to milk the best performance out of it. The X-Terra's can be used to cover a site at a little faster pace, while at the same time allow you to slow down when you get into a target-rich area.
The Sov. GT is better balanced by design than the previous models with their forward-mounted control housing. They are usually a bit nose heavy, too, due to the coil's weight. The X-Terra's are much lighter, and definitely much better balanced.
The ergonomics of the Sovereign GT requires a two-handed operation to adjust any controls and switch into the Pinpoint mode. The X-Terra's have an excellent display face with touch pad adjustments that are easily within thumb's reach of the operating hand.
Battery access of the X-Terra models is also superior, in my opinion.
For the money, I can't help but feel that the new X-Terra 50 is the best deal, and I'd probably put the X-Terra 30 the second best deal compared with the Sovereign GT. For many, however, the X-Terra 30 will be the best deal because it will be much easier to use since it has a preset Ground Balance and ample controls w/o getting too intimidating for some hobbyists.
► And which is just better? For ME, the X-Terra 50 is 'just better' because it is lightweight, well balanced, provides very good user control, and has some features that I personally like. These include the target volume setting and Ground Balance. Also, I like the 4-tone audio and ability to hear the audio and see the TID readout in an all metal accept motion mode. Another thing that helps me afield and makes the X-Terra "just better" for me is the ease of adjustment, the TID readout, and the 18-segment notch discrimination for those times when I can use it to my advantage.
I suggest you consider what you have and/or have used, make a list of what you're looking for, then review what the models have to offer. Next would be a trip to a dealer to get them in your hands and give them a try.
Enjoy the detector shopping and then enjoy some fun time afield!
Monte