Opinions vary, but some of the reasons I like the X-70 over the Cortes are:
the X-Terra display (as well as the information displayed) is much larger than the Cortes. Due to the much smaller display, as well as it being quite reflective of sunlight, I have difficulty seeing the Cortes display without bringing it up toward my eyes.
The Cortes' control knobs are located in such a manner that I seemed to keep bumping them when I would stop to dig a target. The pads on the X-70 are easily accessible and since they are flat, they are not prone to getting bumped.
The X-Terra offers Auto GB, Manual GB, Tracking, Beach mode and each of these are fully adjustible so you can tune the detector for any particular site. The Cortes offers Auto and manual GB in the all metal mode. But you will have to hunt with their factory preset GB in the discrimination mode.
The X-Terra allows the user to program the audio response for one tone, two tones, three tones, four tones or multi-tone mode.
The X-70 offers notch discrimination with two digit notch segments. Any notch or combination of notches can be set as accepted or rejected in any one of the three Pattern modes. The Cortes has variable discrimination with a single adjustible "notch window".
The X-70 TID provides different number information on a variety of coins. The Cortes lumps all silver and copper coins in at a TID of 95. But it does add a block display to help ID targets and has a Sum Mode that "averages out" the target's identification with multiple sweeps. TID Stability on the X-70 allows a more stable TID in highly mineralized soils.
The X-70 The Cortes' audio level in all metal mode is not as "loud" as in discriminate mode. The X-70 offers separate volume settings for use with or without headphones.
The X-70 will operate longer on it's 4 AA cells than the Cortes operates with it's 8 AA cells.
The Cortes operates at 10 kHz. The X-Terra 70 can operate at 3 kHz, 7.5 kHz or 18.75 kHz.
The Cortes has 8 coil options available. Different sizes and shapes. However, they are all the same 10 kHz. The X-70 has 8 coil options available. 6-inch concentric at 7.5 kHz, 6-inch DD at 18.75 kHz, 9-inch concentric at 3 kHz, 9-inch concentric at 7.5 kHz, 9-inch concentric at 18.75 kHz, 6 X 10 DD at 18.75 kHz, 10 -inch DD at 7.5 kHz and the 10-inch DD at 18.75 kHz.
The Cortes offers VCO audio in the all metal mode. The x-70 offers programmable audio frequency in their Prospecting mode, with adjustible Iron Mask.
Everyone's soil and hunting syles are different. But based on my comparisons, and due to many of the reasons listed above, the X-70 is at least 20% deeper on coin sized targets. JMHO HH Randy