I've used all of the Note series and OP phones except the OP2
These days, software experience and performance are the main reasons to get the OP6T. OxygenOS is one of the best Android experiences around. Fast, relatively clean, but with enough customisation options and features inside. They are relatively okay with software updates these days too. To me the software on the OP6T gives a better experience.
The Note9 has the better overall hardware package to me. It's not just better hardware than OP6T, but it has better hardware than most phones in the market still. It has one of the best speakers I've heard on a phone, one of the best screens I've seen on a phone, the camera is Top 5 for sure (though I prefer OP's front camera more), it has IP68, it has S-Pen (which is quite nice to use if you draw), it has headphone jack, it has wireless charging too, as well as some smaller perks like Samsung Pay. BUT... the software experience is less than ideal for sure. Because it's quite laden with Samsung's customisations to the UI, features, apps, etc... It doesn't perform as well. It has useful features of course, but it's just a bit overwhelming. The Exynos chipset in the Note9 also isn't as good as SD845. Then Samsung is not that good at pushing out software updates, especially here in Singapore.
So it really depends on what you're looking for. It's a tough question for sure, especially now Qoo10 having $110 off $800 voucher, so can easily bring the price of the 6GB Mirror Black version down to sub-$800 prices. haha.