I have the regular Pixel and just picked up the S8+ yesterday. I thoroughly enjoy the vanilla android experience but the lack of features associated with the Pixel was not evident until I got the S8+. I love both devices as each appeals to a different audience. Right now I have no intentions of getting rid of the S8+ but will possibly sale the Pixel.
Pixel is the only recent Android flagship device where software and security are at the forefront and where privacy and security are taken seriously. I strongly prefer the metal body over the glass body. Camera is marginally better on the Pixel XL than the S8, though each is strong at their own things.
So to my assessment the Pixel XL wins over the S8 on build materials, software, security, commitment to updates, camera, charging technology, design, lack of bloatware and OEM support.
The S8+ then wins on display accuracy and on newer SoC.
They're tied for all intents and purposes for my usage on water resistance, audio, battery capacity (<1.5% difference), build quality and network support.