The 8 Plus is a sidegrade while the X is a upgrade. The X has a higher quality but a smaller oled display. It's better in every respect than the 7 Plus. The 8 Plus isn't. Better to sacrifice screen size for the bezelless experience. Those bezels look horrendous by 2017 standards.
People go for the smaller phone as it has the most upgrades. The 8 Plus is a 7 Plus with a newer processor and True Tone. The X has a bezelless oled display, FaceID, much better cameras, a more premium stainless steel design, a much better optimised OS for it, OIS in both cameras. Also doesn't have home button delay.
Also I believe all iPhone users before the 7 Plus should be included in this. The 7 Plus is an old phone now because that's how it performs on iOS 11.
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I start using my 7 Plus at 7AM-8AM and it’s running on fumes by 3PM on iOS 11. On iOS 10 it was easily lasting one full working day on a single charge