I have all three models and, as one of the Field Testers, have made many comparisons. As far as discrimination is concerned, both models have notch discrimination and allow the user to set any target to either be accepted or rejected, based on the conductivity of that target. The X-70 offers four ferrous notch segments and 24 conductive notch segments. The X-50 offers 3 notch segments and 15 conductive notch segments. Although both detectors will detect the same targets, the X-70 offers 50% more target notches, allowing the user to better determine the target prior to digging.
As far as depth is concerned.....In simple airtests, the depth differences are minimal if any. However (in addition to understanding how to properly operate your detector) properly adjusting your detector to a specific site just might be the difference between having a successful hunt and going home empty handed. As such, the X-70 will allow you to "auto tune" and "fine tune" some of the adjustments to a greater degree. The X-70 offers AutoGB, Auto NC, Tracking, Beach mode, Prospecting mode, more audio options, more Pattern modes, narrower notches, refined sensitivity settings, and a few other nice features.
In my opinion, if you are a land-locked coin hunter, you will do well with either. If you are a prospector, a beach hunter or someone who appreciates always having the "right tool for the job", you would benefit from the additional features of the X-70. (such as using Prospecting mode for target sizing, 3 kHz concentric coil for old silver and copper). Personally, I am a land-locked coin hunter and I think those two features alone are worth the price difference. But again, that is just my opinion. HH Randy