The main differences I noticed between the 8 Plus and X with regards to cameras are:
1) X has a faster telephoto lens (F2.4 vs F2.8, which doesn't sound like much but is actually fairly significant).
2) X has OIS on the telephoto lens (8 Plus has OIS on the wide camera only) - low light will be better on the X, and presumably video stabilization will also be better when zoomed in
3) X front-facing camera has addition AR features due to the additional sensors
4) X front-facing camera has Portrait Mode.
The sensors seem to be the same. They both also have 4k 60fps video recording, Portrait Lighting on the rear camera, etc.
Personally, I think the camera differences are pretty significant. With both OIS and a faster aperture on the telephoto lens, the X should have noticeably better photo and video quality on the telephoto lens. And the front-facing camera features are pretty significant as well.