I'm assuming the OP is asking from the position of owning a Max and not simply a comparison of the two phones.
If I had a Max and were outside the return window, I would probably keep it instead of selling it and buying a new XR. I have had a XR for over a month and like it a lot, but the primary reason I bought it was its value. I didn't see the XS and Max worth the premium difference over the XR.
However, selling a Max would eat into that difference, so for me I'm not sure it be worth the process. Now--if the size is too big or the owner is sensitive to PWM or has a buyer and won't lose much in the sale, then switching is another matter.