much depends where you are detecting and what you are hunting for. I use them both and each offers advantages in the sites I detect. I like to hunt for old coins at old sites, with "old" being relative to where I live. I prefer the X-TERRA for open fields due to it's lighter weight, variety of coils and being able to sweep it more quickly. In a given period of time, I can cover a lot more area with the X-TERRA than I can with the CTX. I prefer the CTX in trashy areas, due to it's ability to identify both ferrous and non-ferrous targets simultaneously. And I can separate targets with more accuracy on the CTX, by enabling Target Trace/Target Trace Pinpoint. Each model allows for several audio tone options. But being able to assign a tone to specific target groups on the CTX is very beneficial for those who appreciate being able to hunt by ear. To a degree, depth is determined by soil composition, detector stability, coil design, and the "harmonious relationship" between the proper level of Sensitivity and reducing the effects of mineralization. Both detectors offer "smart coils". The X-705 offers Auto Tracking with GB (+ or -) offset. The CTX doesn't typically require GB due to FBS2. The X-TERRA has adjustable Sensitivity. The CTX offers Auto Sensitivity with the ability to offset it either + or -. When properly adjusted and using comparable (size and design) coils, I don't find a measureable difference in "raw depth of detection" between the two.
"Bells and whistles" aside, there are more similarities in performance than some might think. They both operate with a great deal of stability, and they both hunt deep. As with any detector, understanding the functionality and applying that knowledge in the field makes the difference between enjoying a successful hunt or coming home frustrated. JMHO HH Randy