This sunset shot looks great on both phones: The colors in the sky look vibrant, while the city on the bottom looks sharp despite the challenging light. Apple added a Smart HDR feature in the XR, XS and XS Max phones that brings out more highlight and shadow detail in photos.
Photos and videos at the same magnification will look sharper on the iPhone XS than on the iPhone XR which relies on software exclusively to achieve a closer shot. In this shot of the lanterns at 5x, you'll notice the one on the right looks shaper with better detail on the gold strings at the bottom.
The iPhone XR only has one wide-angle lens, which means its relying on software alone to separate the background and foreground. Edges around the hairline of the subjects look a bit more abrupt, but the subjects look sharper than on the XS.
The iPhone XS uses the second telephoto lens to capture the subject. It uses information from both the wide-angle and telephoto lens, combined with software, to discern what to keep in focus and what to blur out in the shot. You'll notice the transition from subject to background looks smoother than on the XR and the image has a softer look.
But if you come in for a closer portrait on the XR the wide-angle lens can cause the subject to look distorted. You'll notice his facial features are different dimensions and look unnatural when compared to the previous shot from the XS.
Getting portrait mode to work on the XR is hit or miss
Portrait mode on the iPhone XR didn't activate fast enough to capture the toddler in motion, so only the XS was able to produce the blurred background effect. This happened a lot, and I had to adjust my distance constantly to get it to work on the XR.
Here's another pet portrait from the XS that the XR wasn't able to get. Apple may add this feature to the XR with a future software update, and you can currently install a third party app that will allow it. But for now you'll get a "no person detected" sign on the camera interface if you try it on anything other that a human face.
The exception to this rule is when there's a person in the shot alongside the pet. When I put my dog next to my toddler in the frame, the iPhone XR was able to capture them both and correctly blur out the background, but it took a lot of adjusting, and a lot of patience.
Portrait mode on the iPhone XR really shines in low light. Because it's using the main lens with the wider aperture, it's able to let in more light in dimly lit scenarios. If you can get past the slight distortion.