It depends.
I have a huge back catalog of 360/One games on digital and I'm gonna need a bigger hard drive than 512GB. However, I found out my 2TB external hard drive will be fine for running non-Series S/X games (those require the SSD drives). So if I'm only putting Series games on the internal drive, I'll be fine.
My thing is if I can get on Xbox All Access, I'll get an X. If I can only just buy outright, I'll get an S. I have a 39" 1080P TV that I'm currently happy with and am not looking to pay $500 upfront when $300 will do fine.