I really like the G6 - perhaps because I'm biased and have had the G3; so I like LG's take on Android and the LG Skin. I also like the G6's bezel-to-body ratio: it's "big," but still seems like someone with small hands like myself could find my way around it easily. I also like its price point and the silver option seems gorgeous. I've heard it can be a little reflective in different lights, but ah well. And their double tap to unlock feature is probably one of the best mobile device innovations ever made (in my opinion, of course).
The S8 line-up is awesome, too. The camera seems great, the colour choices are beautiful, and it sounds like a fast and smooth experience. Oh, and no home button makes me very happy. My only complaint about it would be its price point (but it's a flagship, and Samsung, so I understand the price tag); and that it *may* be a little too big for me. But I can't make any difinitive conclusions on that front until I actually hold it in my hands.
If I had to choose between the two, right now, I'd want the G6. I can see myself keeping it for a while and I really think their design is innovative. My only gripes about the G6 are LG's decision to 1) take out the notification LED and 2) their decision to forego the back button feature.