I think you have to put the comments "good mid-range detector" into perspective. Those who state the X-Terra is "mid-range" might be making that comparison to the Explorer. If so, they might also compare your brother's MXT to a DFX and refer to the MXT as a "mid-range detector". Personally, if it hunts deep, is sensitive to small targets, separates trash well and provides sufficient discrimination characteristics, I don't care what they call it.
I have the X-Terra and the MXT. I also have an Explorer and an XL Pro. And an Advantage and ......well, you get the picture. I find the adjustable discrimination capabilites of the MXT to be better able to identify targets than the notch characteristics of the X-Terras. But honestly, not as good as the Explorer or the XL Pro. Notice I said identify the target, not locate the target. I believe the X-Terra hunts every bit as deep as the MXT, deeper than the XL Pro, and is much lighter weight than any of those others for surviving those all day hunts. The X-Terra can be worked much faster than the Explorer, faster than the MXT, but not as fast as the Advantage or the XL Pro. The MXT offers 2 tone target ID (in Relic mode) and the X-Terra 50 offers 4 tone audio ID in all modes. The MXT offers a preset notch pull tab rejection and the X-Terra 50 offers 18 separate notches that you can either accept or reject. The Explorer can be programmed to hunt just about how ever you want it to. Except fast! A big factor, if you are willing to look down the road a few months, the X-Terra 50 will allow the flexibility to change the frequency of the detector by simply adding the 18.75 kHz coil. Not the X-30, just the X-50.
So, which one you buy is a decision you will have to make on your own, based on the priority of your needs. Weight, balance, depth, sensitivity, separation, visual target ID, audio tones, notch vs adjustable discrimination, manual ground balance vs automatic ground balance, battery life, etc. Study them all and determine what is important to you. If you have any specific questions, I can try to help answer them for you. HH Randy