We contemplated a Blind camera shootout between these two, but after looking at the images we abandoned the idea. The photos are just too similar. As in, top-quality, super-awesome, class-leading, but still too similar.
That's not to say that there aren't things that HTC and Samsung have done differently, no. For example, we observed more vivid colors in the U11's samples across various scenes and shooting conditions. Its shots have that extra bit of pop and next to them the S8's photos are (just marginally) less contrasty.
Foliage is rendered expertly by both, perhaps with a notch more sharpening on the S8+ than the U11, but again - it's the slightest of differences.
The two phones are quite evenly matched in dynamic range too, though the second sample above might lead you to believe that the HTC is superior - it just picked a brighter exposure so that's why you have more detail in the shadows.
Look closely and you'll see that the S8+' noise reduction has done away with some of the fine detail - the building in the distance to the left of the Transamerica Pyramid is one example where the HTC has retained more definition. But seriously, that's splitting hairs.
Of course, feel free to pixel-peep in the full resolution images below.
Somewhat more pronounced differences emerge in low light. Apparently, the Galaxy S8+ has more competent stabilization, or at least the auto mode is so tuned that it allows for slower shutter speeds. The U11, on the other hand, tends to crank up the ISO before dropping the shutter speed and that results in some more noise and a softer image, as a general rule.
However, the HTC has a visible advantage in color reproduction, its lowlight images having noticeably more punch than the S8+'. The Galaxy's shots are a little more muted, though not half bad if examined on their own - it's just that the U11 is more pleasing to the eye.
You can always examine the original photos and make your own conclusions.
The front-facing cameras are nothing alike - the two makers have taken different approaches here. HTC went for the megapixel buzz and fitted a 16MP shooter, while Samsung opted for a minor bump to 8MP from the 5MP snappers of older Galaxies.
The U11 doesn't have as strong an advantage in captured detail as the gap in resolution would suggest - the S8+ is doing fine. High-frequency detail in the hair is undeniably better resolved on the U11, though.
The two phones handle skin tones truthfully, no complaints here. Neither selfie camera is a dynamic range champ, but we don't really expect them to be.
In low light the Galaxy S8+ holds on to color better, with the U11's shots losing saturation. Even in the dark U11's selfies are more detailed - with the screen flash enabled, of course.