Both of these phones have powerful cameras on the back, but how they work is very different. That's because Samsung's brought something completely new to the table in 2018 while Google continues to use software and AI to do amazing things.
Samsung introduced an all-new variable aperture camera on the S9, and now the Note 9. What that means is the sensor can change from f/1.5 to f/2.4 apertures for the best possible photo in any situation, speed, or lighting. Just like how the human eye adapts to its surroundings and light, Samsung's camera can do the same thing. Basically, letting in more light for better photos.
If it doesn't need a lot of light it uses a faster aperture for crisp, clear photos. As a comparison, the Pixel 3 is only f/1.7 and it can't change. Plus, Samsung's Note 9 has dual rear cameras.
That said, Google's Pixel 2 can still take amazing photos. Some of which are better than what the Galaxy S9+ or iPhone X need two cameras to achieve. Google's machine learning and AI makes it take great photos. And now, the company confirmed the improved 12.3 MP camera in the Pixel 3 and 3 XL is even better. They're using more software tweaks, enhanced HDR modes, and other Pixel Visual Core processes for the best photos ever.
Basically, both of these phones will have the two best smartphone cameras of 2018, so buy either one and enjoy the results. Google might have the better shooter, but Samsung's Note 9 gives you more options with variable apertures and a zoom lens.