In broad daylight the two phones capture excellent images that are surprisingly hard to tell apart at fit to screen magnifications. Only minor differences in color rendition can be spotted from afar - the Xperia's reds are deeper, while the Galaxy's are a touch more orange, and yellows are a little warmer out of the Xperia, and ever so slightly greenish on the Galaxy. Oh, and the grass is often greener on the Xperia's side, too. That said, a lot of these could be due to the scene mode the Xperia XZ2 decides to pick in Superior Auto.
Click on the 1:1 button (or use the keyboard shortcut in your viewer of choice) and you'll immediately see the difference - a difference that is present in every shot. The Xperia's images have some extra fine detail that the S9's lower-res sensor can't quite resolve.
The flipside is that the Galaxy's photos are significantly smoother, with noise competently and almost completely weeded out. At the same time noise is visible on all of the Xperia's images, even at ISO as low as 40. Just don't get us wrong, it's not that bad at all, it's just that the S9 is remarkably clean of any noise.
The Galaxy S9's Auto HDR (or 'rich tone' as they call it) does a better job with high-contrast landscape scenes than the Xperia. The proficiency of Samsung's HDR algorithms aside, there's another factor at play here, and it's Superior Auto's tendency to pick the 'Landscape' scene mode over the HDR-like 'Backlit'. That typically yields a brighter exposure and blown highlights.
The Noise vs. Detail trade-off aside, it's a very close race between these two in good light, which is a pleasant surprise, we have to admit. Let's see how things look in less than ideal conditions.