The design of these two phones is similar on the front, but vastly different at the same time. Aside from the small notch in the screen, the OP6T is all screen on the front. Giving you a more futuristic-looking device than the Note 9. As great as it is, the Note 9 has a two-year-old design.
The biggest design difference is that the fingerprint sensor is inside the screen on the OnePlus 6T. It's an in-display fingerprint sensor. Something we're expecting from Samsung with the Galaxy S10 in 2019. OP beat them to the punch.
Other differences you need to know about are the 3.5mm headphone jack and expandable microSD storage slot on the Note 9. The OnePlus phone doesn't have either one. It doesn't have wireless charging or water-resistance either. Those all give Samsung a leg-up.