I used to use the 6S but I find the 8 screen is much much better. The colors and True Tone make it amazing.
You're comparing two entirely different displays that are calibrated differently. The iPhone 6s had a bluer/starker tone compared to the iPhone 8 that has a much warmer/dimmer tone to it.
I think that's turning TrueTone on, plus the wider color gamut. .
The iPhone 8 has been confirmed/remeasured to have a dimmer display even over the iPhone 7, including the iPhone 6s. Which is speculated Apple did that to conserve power. It's not due to True tone, assuming that it's turned off.
Read through this thread as it discusses other comparisons as well of the iPhone 8.
https://forums.macrumors.com/threads/iphone-8plus-screen.2096082/#post-25616402