With so many cameras on board both phones have faux bokeh modes. The one on the Galaxy is more versatile, as we established, letting you shoot with either the telephoto or the main camera, while the Xperia only uses the telephoto.
But even setting aside the Galaxy's extra camera capability, looking at telephoto-vs-telephoto shots only, the S10+ is the significantly more proficient portrait taker. Its subject detection is excellent and the phone draws borders between subject and background with a precision the Xperia can only dream of.
The Galaxy's main camera portraits are even better, though you may be reluctant to get so close to your subject if there's no need to. There are two main benefits to using this camera - selfie portraits with the rear cameras let you fit more than just a face thanks to the wider FoV, and you'll also get better results in dim light.
The Galaxy maintains its lead when shooting non-humans in Live focus mode once again delivering better edge detection, whether you're shooting with the telephoto or the main camera.
Winner: Galaxy S10+. Whether you're shooting people or stuff, the Galaxy offers superior edge detection, plus the ability to shoot with either camera means improved low-light performance and rear-camera selfie portraits.
For selfies, the Galaxy S10+ has the superior hardware, and more of it too. It shows in the photos, particularly the selfie portraits, where the extra sensor delivers much better depth detection and consequently more convincing subject isolation.
It takes only a modestly complex background for the Xperia's processing to get fooled and leave random patches in focus, while it also sometimes blurs parts of the face. The Galaxy is a lot more competent and creates nice portraits across most subject/background combinations.
That's not to say the Xperia doesn't take nice selfies with the bokeh mode off. Its images are decently detailed and have accurate skin tones.
The Galaxy tends to give faces a warmer makeover, perhaps a good way to make you look more appealing. Its images are also more detailed though the HDR processing robs them of the finest detail - if you want all your pores captured, keep it off.
Winner: Galaxy S10+. The Xperia takes decent selfies, but the ones out of the Galaxy are even better, and the Sony barely puts up a fight in portrait mode.