I have owned and used many Samsung phones including the S8 and I have owned and used every OP phone since the OP3. The software experience is hands down better on OnePlus. EVERY Samsung phone I have had has become unbearbaly slow after just a few months of use where as I have yet to experience a slow down on any OnePlus phone. Even out of the box, the OP phones runs circles around Samsung. Quite literally the only upgrade from the OnePlus 6 to the S9 would be the camera. Everything else is better or comparable on the OnePuls 6.
I don't use the camera a whole lot so for me the software and user experience is a make-or-break feature for me and Samsung's is still just terrible. OnePlus is also pretty solid about updates.
The main question for you is what do you mostly use your phone for? If you are a more casual user then absolutely the OP 6 is the phone for you. If you really need the best camera you can buy, or if you watch a whole lot of videos on your phone and must have the incredible display of the S9, or if you still need expandalbe storarge, then those are really the only reasons I can see for getting an S9. That being said, the S9 is a solid phone (minus the software experience) and you couldn't go wrong either way. I would still recommend the OnePlus 6.