I went through six Xs Max before giving up. Four were perfectly uniform, two had minimal colour shift, but all had very warm screens and issues with signal strength. They also all had very noticeable PWM. Although I prefer Apple and iOS, I also own the S9+ and the Note 9 (and a number of other phones) - neither have the colour shifting of the iPhone Xs Max and the PWM was less of an issue for me. When viewing images and videos, the colours on the Xs Max do appear more accurate than those on the S9+ and Note 9, which I find to be a little too blue (and I don't mean cool). When viewing webpages, the colours on the Xs Max do not seem accurate. Placing a 'layer' over the Xs Max screen using the display accommodations is not a 'fix' (this is actually a terrible idea).
I have a degree in multimedia/graphic design, and have worked in this industry for twenty years, so I am more than familiar with how colour, screen temperature, and screen calibration works (I am by no means saying I am an 'expert'). I am also very pedantic when it comes to the screens in my devices since that is what you spend the most time looking at. I have no issue with my 2018 MacBook Pro - it is the best and most most accurate screen of any MacBook Pro I have owned. I also have an iMac with no issues.
Yes, OLED is a superior technology to LCD with better blacks, colours and contrast (I am talking about the technology in general here, not just phone screens). However, (for ME) I don't believe that the OLED screens in the Xs series are the best from the devices I have tried. A persons view is very subjective on this (for example, many will disagree on this point with me). If you got a 'perfect' screen (which few do actually exist), then you are one of the lucky few and if that were me, I would be holding on to it!
If the issues with the Xs screens are not a problem for the majority of people, then that is great if they are happy with the device. However, for a number of people, the issues do exist, and as someone who is sensitive to PWM, I cannot use the Xs Max without getting headaches, blurry vision and feelings of nausea. I even went and had my eyes tested last week (which were fine). The weird thing is, I never had this issue with my iPhone X, which unfortunately I no longer own.
In the end I have settled on the Xr. Yes, the 'resolution' is lower, but what a lot of people don't seem to understand is that the PPI also needs to be taken into consideration. A lot of people seem to be making comments without fully understanding the technology (not just in this thread, in general). Does the iPhone Xr look as 'pretty' as the Xs iPhones? Hell no! The bezel on the Xr is more noticeable, the fonts are sharper on the Xs at closer viewing distance, the colours and contrast are superior, and the rear camera is preferable (we now have the Halide app for portrait photos on the Xr). The three Xr I have used have had a very small colour shift - but not to the extent of the Xs. The Xr screens I have seen do not exhibit the warmth present on the Xs Max (I am not saying the Xr screen is cool, because it is not).
Would I pick the Xs Max over the Xr if there were no issues with PWM, the warm screen and poor viewing angles, and low signal quality? YES, in a heartbeat. But the reality is, that is not going to happen for ME, the screen on the Xr is just much easier on my eyes. So, although I can more than afford the iPhone Xs Max, I cannot compromise on the issues that are important to ME - and so the Xr is the choice for me at this time. Will I try again next year when Apple releases their 2019 iPhones? Of course.
To answer the OP - yes, the screen on the Xs does appear to be much sharper than that of the Xs. It is mostly noticeable when viewing text at a closer distance - and you will see the difference if you have the two handsets side by side. I think Apple have done a great job with the screen in the Xr, but for anyone deciding between the Xr and Xs - they should view both in person.
At the end of the day, it is just a phone and there are other issues more significant in life.