The XR is better value wise on price. But the XS will sell better in the long run.
I disagree with your second sentence. The XS will not sell better in the long run, why? Because it’s considerably priced higher, where as the XR is $250 cheaper, and will likely get discounted if it remains in Apples lineup when version two of the 2019 XR launches. Consumers will see the similar form factor and Face ID between both the iPhone XR and Xs, which they will gravitate towards the cheaper price of the XR.