Better doesn’t really mean much in this context. What do you need this phone for? Will you use it for work? Gaming? do you need a very good camera? Is battery life important for you? Do you value repairability?
Both phones are good, and as you said, they’re around the same price range, which usually means the quality is similar.
The pixel 6 is a good device, and graphene only makes it more worth, being one of the best custom roms out there for privacy.
There is no reason in particular I would choose the oneplus over the pixel or any other phone around the same price range.
I suggest you buy a refurbished one, their condition is usually very good and you save quite a bit of money.
If you are in EU I strongly recommend www.fairphone.com/en/They’ll launch the FP5 in some time, consider that too