I have now had the brand new space grey iPhone XS Max for about four days and so far I am really enjoying it. I upgraded from an iPhone 7 plus so the screen has been the largest change. I live on my phone during the school day using it for checking laptops in and out through the day and looking up IP addresses for classroom printers having that large screen has been so nice!
During band rehearsals I am conducting rehearsals from scores on my iPad and in the past I have used tuning and rhythm practice apps with the students on my iPad. It has worked well but requires a lot of quick jumping around and with students in the room and my attention being in many different areas at once the jumping around can be awkward. The larger screen iPhone will now make this much easier as I can have my iPad and iPhone sitting on my stand and can project from my phone and jump between the 2-3 apps I use during rehearsals and can keep the scores up on my iPad.
I never used an iPhone X but from just looking at the specs if you have an X there is really no point to upgrade, the only real reason is if you want that larger phone size otherwise I would stay with what you have. I will be updating this post after about a month so I can do a thorough review of my use of it in the classroom.