The Galaxy S9 Plus has a 2x zoom lens, while the Pixel 2 XL doesn't.
Keep in mind that the regular Galaxy S9 – the non-Plus version – doesn't have a zoom lens. So if you're looking at buying the regular Galaxy S9, it'll be closer to the Pixel 2 and Pixel 2 XL, which also don't have zoom lenses.
Which smartphone wins?
Look through your photo gallery and see if you take most of your photos in dark settings. If you do, the easy answer is the Galaxy S9 and S9 Plus. And if you find that you zoom in a lot, or you would like to zoom in more than you currently do but zoomed photos tend to look blurry, the S9 Plus and its 2x zoom lens is for you.
Otherwise, if most of your photos are taken during the day or in decently-to-well lit environments, you'll be happier with Google's Pixel 2 and Pixel 2 XL. I prefer the way it shows deep, rich colors compared to the S9, which can sometimes wash out colors and cause photos to become overly bright. The S9 and S9 Plus handle bright situations very well, but just not quite as well as the Pixel 2 and Pixel 2 XL.
With that said, the Galaxy S9 phones offer more versatile cameras, as they take good shots in decent lighting, and they can take much better photos in low-light environments than the Pixel 2 and Pixel 2 XL. And if you're considering the larger Galaxy S9 Plus with a dedicated 2x zoom lens, you'll get better zoomed shots than you would with the Pixel 2 or Pixel 2 XL.
