In ideal light, and when using automatic settings, both the Galaxy S9 Plus and the iPhone X are at the top of their class in terms of mobile photography. This is shot of the lake looks brighter with the Plus.
But sometimes the retouching on the S9 Plus can backfire. In the photo above, the S9 seems to have applied the same filter on my face as it did to my son. On his face it evened out his skin tone and reduced redness, but the same effect on me made my skin turn yellowish.
While the iPhone X has a 7-megapixel depth-sensing camera which is slightly better at figuring out what to blur in the shot. But both rely on software to create the portrait effect, so neither is as good as the rear-camera. In other words, don't expect perfection.