The Galaxy S9 uses a better screen technology than the LG G7's.
Samsung uses AMOLED displays, a type of OLED display technology, for its Galaxy lineup, and it's superior to the LCD display on the LG G7 in nearly every way. Contrast is more dynamic, colors are richer, and it doesn't use up as much battery, which is useful for features like the "always on" feature that shows you basic information on the screen without even waking the screen's display.
The Galaxy S9 has a better design than the LG G7.
It's a matter of personal taste, but it's hard to argue that the Galaxy S9 doesn't look better than the LG G7. The curves and edge-to-edge display stand out in particular. And there's just a slightly more premium look and feel about the Galaxy S9 that the LG G7 just doesn't quite match.
The Galaxy S9 has stereo speakers.
Stereo is better than mono, period. Yes, LG gave the G7 a great — and extremely loud — speaker, but it still delivers mono sound that's not as dynamic as stereo for music and videos.
The Galaxy S9 has a better camera.
Samsung simply has some of the best smartphone cameras with its latest Galaxy lineup. The LG G7 also has a good camera, but I never trusted it as much as I do the Galaxy S9 during my day-to-day testing.
If you often want to zoom in for your photos, the Galaxy S9 Plus is the better choice.
The Galaxy S9 Plus' secondary lens is a 2X optical zoom lens that's better than the digital zooming on the LG G7 (and any other phone that uses digital zoom, like the Google Pixel 2 for that matter).
Bonus: The Galaxy S9 has a built-in heart rate monitor, which some might find useful.
I don't really count the built-in heart rate monitor in the Galaxy S9 as an official "thing" that's better than the LG G7. I used it once to see how it worked and never used it again because I had no use for it. Other fitness fiends out there might think differently, so it's good to know that the S9 has that feature, at least.