I have used both. The screen on the Plus is wider (by 7 mm?) and keyboards, letters, and pictures are generally larger.
PhoneArena has a good discussion about the display, pixel area. But there are tons of good discussions about how the traditional aspect ratio of the Plus can provide for a larger viewing area for videos, pictures etc.
If you go full screen with the X, the outside portion of the content generally gets cut away (unless you are viewing widescreen) and occasionally written annotations on YouTube videos get removed.
If your vision is poor, the Plus will provide for a better experience for text.
After using both, it was hard to tell the difference in display quality (pixels). The X has a more contrasty display and the images are better, but they are generally smaller. The blacks and white are better but the letters on websites are generally smaller.
I like the Plus experience a tad better. But my eyes are not what they used too be and I am red, green color blind so my situation my not be applicable to you.