These phones run close on features, performance, battery life, price and everything in between.
If you want bleeding edge design, you should get the Galaxy S8. The Infinity edge display is one of the best displays on a smartphone ever, and the near bezel-less design means the phone isnt too large even in one hand.
Youll also get wireless charging and full waterproofing.
However if you want a smaller 5in phone, love stock Android and want Android and security updates with what is debatably still the best camera in a smartphone, then go for the Pixel.