Source: https://phalaps.com/differences-and-similarities-between-samsung-galaxy-s9-and-samsung-galaxy-s9/

As being noted, the major differences between Galaxy S9 and Galaxy S9+ are the screen size, camera, RAM and internal memory, body build and dimensions. The Samsung Galaxy S9+ features a 6.2 inches display and Super AMOLED Capacitive touchscreen with 16M colors screen while Samsung galaxy S9 comes with a 5.8 inches display and Super AMOLED Capacitive touchscreen with 16M colors screen, S9 and S9 plus has 1440 x 2960 pixels, but S9+ comes with ~ 531 PPI while S9 comes with 581 pixel density.