Face ID - It works really well and makes the idea of touching your device to authenticate and needing a finger sized sensor to do this seem dated. I like the idea that your phone is aware of when it's owner is using it and authenticates on it's own.
Display - I like the way the glass seems thinner than the 8 Plus. I'm not an OLED fan and I hate the garish Samsung phone displays put the X display is acceptable. The display on the 8 Plus is really good though and side by side with matched brightness they are close. Both displays have their issues, the X has a slight pentile fuzz and the 8 Plus has less legibility. If only the X display was 5mm wider....
Size - This is both a complaint and a feature and it's clear that Apple chose this size so as not to alienate those who need smaller phones, and also to have something that would draw in Plus users. For me this means the first time in ages that I can fit my phone in any pocket I have.
Build - The X is gorgeous, and the build makes the 8 Plus look very "glass sandwich".
So compromises, pluses and negatives.