Although pretty similar in size and capability, there are some differences between the XS and XR models that could make a difference in the phone you choose.
The XS has a stainless steel frame which feels a bit more premium than the 7000 Series aluminium frame of the iPhone XR
The XR lacks the pressure sensitive 3D Touch technology which the XS offers
The XR has a downgraded LCD screen as compared to the OLED screen of the XS which results in substantially lower native resolution which means your colors won’t be as bright or as sharp as the XS
The XR has a slightly larger display (about a 5% larger) than the iPhone XS
With an IP68 rating, the XS can be submerged for up to 30 minutes in two metres of water while the XR’s IP67 rating allows it to be submerged in only one metre
If color is a deciding factor, the XR, which is available in white, black, coral, yellow, blue and red might be your choice since the XS is only available in silver, space gray and gold
Price is probably the biggest difference with the XR retailing for $749 versus the $999 (64 GB) for the XS
The iPhone XR has a single rear camera, and the XS has a dual rear camera. The XR also lacks a telephoto / zoom lens which is a pretty big difference if you take a lot of photographs
In summary, if your looking for the best Apple has to offer and photos are of prime concern, choose the XS (or the XS Max, if you prefer a larger screen and additional battery life)
If you’re looking for the most affordable iPhone with many but not all the bells and whistles, choose the XR