I had a lot of difficulty deciding between the XR and XS. See this thread. Here are my observations after one week and a thousand photos later. I hope it can help others making their decisions 
Size
The main reason for choosing the XS over the XR was the size for one-handed use. I am using the XS with a Ringke Onyx case. It’s a great case with good protection but it does add some bulk. I don’t think I would like to use an XR with a case.
Zoom lens
I didn’t have a zoom lens on my previous phone, the 6S. I didn’t know if I would need the zoom lens. Two reviewers (Daring Fireball, Austin Mann) mentioned that they really miss the zoom lens on the XR. After taking a thousand pictures I think it’s extremely useful. I generally prefer the shots with the zoom lens because most subjects simply look better with a nice crop. And it's so convenient and fast to zoom with the "2X" button.
Portrait mode
One of the advantages of the XS over the XR is the ability to take portrait pictures of non-human subjects. Again I didn’t know if I’d really use portrait mode a lot for that. But it is a great feature despite the annoying distance requirement. I have taken tons of dog pictures. It is more fun to be able to take pictures with bokeh.
Food pictures
I love reviewing restaurants and taking food pictures. I was hoping it could replace my Canon G7X Mark II for this purpose but unfortunately it can’t yet. One very annoying aspect is that you need a lot of distance for the portrait mode to work so I end up leaning all the way back over my chair. It’s quite uncomfortable at a restaurant where you don't have a lot of space. Also the result is a picture that is too much zoomed out. If I crop the image, it’s still pretty good for Instagram. However I'd still bring my camera if I want to take pictures for my food blog.
256 GB
One of the more difficult decisions was whether I should pay an extra 150 USD to upgrade from 64 GB to 256 GB. At this moment I have used 47 GB with the photo library being 6.2 GB. The new HEIC format results in smaller 2.5 MB pictures so I could take a few thousand more pictures with 64 GB. I have started taking my video clips in 4K because of the 256 GB. 256 GB is definitely overkill for me but 64 GB is a bit tight so I don't regret the upgrade in the end (a 128 GB option like for the XR would have been perfect).
Conclusion
The XS is a joy to use compared to the 6S. Although the 6S didn't feel slow, the XS is so much faster and very smooth. FaceID and gesture controls are a big improvement too. I really wish the battery life was better. I get about 5 hours screen on time which gives me battery anxiety at the end of the day (if I don't charge it in the afternoon).
The camera is extremely fast which makes it more fun to snap a picture. I love the fast zoom and being able to use portrait mode for all types of subjects. But I don’t like the finicky process of taking portrait photos and low light images are too noisy.
Overall I'm glad that I got the XS over the XR for the camera. I didn't know if I needed the extra features but if you love taking pictures then go for the XS.
Size
The main reason for choosing the XS over the XR was the size for one-handed use. I am using the XS with a Ringke Onyx case. It’s a great case with good protection but it does add some bulk. I don’t think I would like to use an XR with a case.
Zoom lens
I didn’t have a zoom lens on my previous phone, the 6S. I didn’t know if I would need the zoom lens. Two reviewers (Daring Fireball, Austin Mann) mentioned that they really miss the zoom lens on the XR. After taking a thousand pictures I think it’s extremely useful. I generally prefer the shots with the zoom lens because most subjects simply look better with a nice crop. And it's so convenient and fast to zoom with the "2X" button.
Portrait mode
One of the advantages of the XS over the XR is the ability to take portrait pictures of non-human subjects. Again I didn’t know if I’d really use portrait mode a lot for that. But it is a great feature despite the annoying distance requirement. I have taken tons of dog pictures. It is more fun to be able to take pictures with bokeh.
Food pictures
I love reviewing restaurants and taking food pictures. I was hoping it could replace my Canon G7X Mark II for this purpose but unfortunately it can’t yet. One very annoying aspect is that you need a lot of distance for the portrait mode to work so I end up leaning all the way back over my chair. It’s quite uncomfortable at a restaurant where you don't have a lot of space. Also the result is a picture that is too much zoomed out. If I crop the image, it’s still pretty good for Instagram. However I'd still bring my camera if I want to take pictures for my food blog.
256 GB
One of the more difficult decisions was whether I should pay an extra 150 USD to upgrade from 64 GB to 256 GB. At this moment I have used 47 GB with the photo library being 6.2 GB. The new HEIC format results in smaller 2.5 MB pictures so I could take a few thousand more pictures with 64 GB. I have started taking my video clips in 4K because of the 256 GB. 256 GB is definitely overkill for me but 64 GB is a bit tight so I don't regret the upgrade in the end (a 128 GB option like for the XR would have been perfect).
Conclusion
The XS is a joy to use compared to the 6S. Although the 6S didn't feel slow, the XS is so much faster and very smooth. FaceID and gesture controls are a big improvement too. I really wish the battery life was better. I get about 5 hours screen on time which gives me battery anxiety at the end of the day (if I don't charge it in the afternoon).
The camera is extremely fast which makes it more fun to snap a picture. I love the fast zoom and being able to use portrait mode for all types of subjects. But I don’t like the finicky process of taking portrait photos and low light images are too noisy.
Overall I'm glad that I got the XS over the XR for the camera. I didn't know if I needed the extra features but if you love taking pictures then go for the XS.