The Note 10 Plus has a 6.8-inch AMOLED screen with 3,040 x 1,440 pixel resolution, while the Note 9 has a 6.4-inch AMOLED screen with 2,960 x 1440 pixel resolution.
In layman's terms, these are both superb displays with bright, accurate colors. The Note's famous rival, the iPhone XS Max, has just an OLED screen with 2,688 x 1,242 pixel resolution.
Unless you're holding the phones right next to each other, you likely won't notice the difference in screen resolution. What you will notice, however, are the bezels. The Note 9 has very visible bezels on the top and bottom of the screen, while the Note 10 Plus, with Samsung's signature "Infinity-O" display, is about as bezel-free as you can get. If you covet a bezel-less experience, the Note 10 Plus is the phone for you.
The 10 Plus does have a camera notch in the center of the top of its display, which is visible when you're watching videos. Check out some photos and make sure this won't be a deal-breaker for you before you upgrade.
Winner:
Note 10 Plus