Tangibles
Samsung Galaxy S9 Plus has a 3.5mm headphone port, while the Huawei P20 Pro doesn't.
Samsung Pay, Google Pay available on S9 Plus, and
just Google Pay only on P20 Pro. So you miss out chances on using Samsung Pay points to redeem like free wireless charging pads (Just ended but is likely to come back soon) or even the recent Chicken Up discounts

Citi customers who are Samsung Pay users can now view and refresh their miles or points balance in-app and in real-time. They also can use their points and miles for instant redemption on transactions made via their mobile wallet using eligible Citibank credit cards.
Alot of exclusive stuff on the S9
IP Ratings
The Huawei P20 Pro is
IP67 certified,
meaning it can be submerged up to 1 meter deep in water for up to 30 minutes, but the Samsung Galaxy S9 Plus tops it with
IP68 certification,
allowing it to be submerged up to 1.5 meters for 30 minutes.
Display
The
Huawei P20 Pro has a 6.1-inch 1080 x 2240 OLED display, while the
Samsung Galaxy S9 Plus has a 6.2-inch 1440 x 2960 Super AMOLED display.
The
S9 Plus has a slightly larger and significantly sharper screen then, plus it’s also curved, while the P20 Pro's is flat.
P20 Pro has a notch in the screen, the Galaxy S9 Plus doesn't, though both phones have small bezels. But it is very odd that P20 Pro has a chin even with the notch. (Defeats the purpose doesn't it)
Camera
P20 Pro has get a
12MP f/1.8 RGB sensor, a 40MP f/1.6 monochrome sensor and an 8MP telephoto sensor.
Samsung Galaxy S9 Plus meanwhile has a
pair of 12MP cameras, one of which offers a variable aperture that can
switch between f/1.5 and f/2.4, and
one which is fixed at f/2.4. That
f/1.5 aperture is a smartphone first.
Both phones' cameras offer super slow-motion video at 720p quality and 960 frames per second HOWEVER what Hua Wei wouldn't tell you.. The P20 pro is actually
incapable of 960 FPS video recording. It is just a
frame interpolation technique used to make it 960,
so it is not true 960 FPS. Clearly seen in the video below and many more
AI modes on the camera are basically called Scene detection that crap has been there for ages,
AI term is all pure marketing right now.