The XS Max will be a huge upgrade for anyone not on the X.
I am on an X and want it.
The form factor of the iPhone Plus has always felt slightly larger than I’m comfortable with. If the Plus form factor is a good fit for you then the increase in screen size will be a huge selling point for the Max.
I basically stopped using my 9.5” iPad Pro after I got the X last year. The X was basically the same form factor as the standard iPhone but removing the bezel added enough additional screen space that I don’t feel like I’m missing anything when I browse the web or use apps in the way I did prior to the X.
I’d advise anyone planning on buying the iPhone XS or the iPhone Max who is also thinking of buying an iPad for the additional screen space to wait on the iPad unless you know for certain you need the features on the iPad Pro. You may find that after you have the XS or the Max that having more screen space is no longer an issue.