I recently went from a 6S to an XS (non Max). I am not a heavy phone user and have been disappointed with battery performance in comparison to the 6S. It doesn't seem to hold up as well, nor does it charge as quickly. From what I've read, the XS Max and XR do better.
I went from an 8 to an Xr. The resolution in terms of dpi is the same on both. In terms of overall pixels it is higher on the Xr, due to bigger display size. Both displays are excellent.
I thought I might miss 3d touch, but with the iOS 12.1.1 update the biggest issue had been fixed: Accessing the context menu for notifications on the home screen. That one now actually feels nicer than on the 8, since the feedback is stronger with haptic touch. Scrolling in notifications with long press on spacebar is slightly worse, since it doesn't allow to mark things via light-release-and-press and doesn't react in some situations. But it's still very bearable. Don't miss the peek&pop functions of 3d touch at all.
The biggest advantage and disadvantage of the Xr: It has a lot bigger screen, is bigger, and feels heavier. It's a tradeoff. I still find it more inconvenient to carry around, and to swipe in from the left side of the screen in order to go back. But the large screen also has it's advantages.