I have owned a couple MXT's and all of the X-TERRA's. When considering both the MXT and the X-TERRA 705, there are several similarities. Coin modes, Prospecting modes, tracking, discrimination etc. I won't deny that both are great detectors. And other then my XLPro (which is no longer manufactured), if I were going to buy another White's detector, it would likely be the MXT. But the reason I don't need the MXT is because I have the 705. Some of the many reasons that I prefer the 705 are......in no certain order..... weight and balance, ability to operate at any one of three frequencies, noise cancel, notch discrimination, multiple tone audio and Tracking GB offset.
The weight and balance difference is pretty self-explanatory. The physical design of the MXT implements technology that White's has been using for 20+ years. Some might say, "if it ain't broke, don't fix it". But technology has allowed manufacturers to package more electronics in a smaller package. The X-TERRA takes advantage of that by designing the X-TERRA control housing to be a fraction of the size and weight of the MXT.
The MXT operates at 14 kHz. The X-TERRA can operate at 3 kHz, 7.5 kHz or 18.75 kHz, depending on which coil you have on. Having the ability to operate at any one of three frequencies is unique to the X-TERRA. Although any of the coils are capable of finding anything metal, each frequency has benefits for specific conditions and targets. If you want to learn more about the application of the various frequencies and coils, I invite you to read the eBook, Understanding your X-TERRA. The link to the FREE book can be found at the top of these posts.
Noise Cancel is an adjustment that choses the optimum operating channel for the particular site you are hunting. If there is any electrical interference in the area, adjusting the Noise Cancel can reduce or completely remove that interference. This allows the detector to perform to it's maximum capability by slightly changing the operating frequency of the detector, nullifying the effects of the electrical harmonics.
The notch discrimination of the X-TERRA allows you to reject any given target(s) and accept all others. With the variable discrimination of the MXT, when you set the level of discrimination, you are rejecting any and all targets with conductive values less than or equal to where you set it. In other words, the X-TERRA allows you to reject bottle caps and still accept nickels. With the MXT variable discrimination set to reject bottle caps, you are not only rejecting nickels, you are also at risk of rejecting various pieces of gold jewelry, old silver coins (3-cent silver) and anything else that has conductive values less than or equal to where you set it to reject bottle caps.
In Coin/Jewelry mode, the MXT offers one tone for all targets. In relic mode, it provides one tone for ferrous targets and one tone for conductive targets. The X-705 offers the user the choice of having one tone for everything, two tones, three tones, four tones or multiple tones. Since there are 28 notch segments on the X-705, in multiple tone mode, you have one separate and distinct audio tone for each of the 28 notches. When using multiple tone audio, nickels sound different than pennies, pennies sound different than dimes, dimes sound different than quarters, quarters sound different than silver dollars etc, etc.
Both detectors offer adjustible ground balance and both offer Tracking. But the X-705 allows the user to set the ground phase automatically, via the software. This provides a precise ground balance for any given location. In addition. the X-705 offers Beach Tracking, which updates more quickly than regular tracking, and a feature called Tracking Ground Balance offset. This allows the user to run the detector with either a negative or positive ground phase, and maintain that "offset" in a tracking mode. What this allows is for the user to have the optimum ground balance throughout the hunt.
As I said, there are many reasons I chose to use the X-TERRA 705. If you have any additional questions, don't hesitate to ask. There are a lot of knowledgeable folks here on the X-TERRA forum. HH Randy
Anyway, I'm hoping to learn from all the experienced people here and I have a few questions. I live in the S.F. Bay area but own land in gold country near Grass Valley, I also have the old Emmigrant Trail running right through my land! So I will hopefully be using it for finding relics and coins on my property but maybe some prospecting nearby as well. 
I guess once I learn the machine, I'll have to use my findings to fund a DD coil.
Anyway he is asking 550 obo. I would not offer more than 450 if it checks out tomorrow. It comes with a sand scoop, small SS trowel, bag and headphones. I'll see if he still has the receipt and a transferable warranty.