We have an X, and have had a couple of S. Love them both, but they are indeed quite different. skip8jj summarized pretty well above, but if you want a lot more text, I described the differences in
THIS thread.
Being a frugal guy (nobody looking at my cars seems to believe me!) I usually start decisions like this by looking at the cheap option. The S is cheaper, more efficient, more reliable, has more range, charges faster, is quicker, and looks better. To me, that should be the "default" choice. My wife and I would love to go back to one some day.
So what does the X offer that might make some switch to the more expensive option? The main draws of the X: available seating for more people, towing, a little extra space (not really much), and better ingress/egress. Are any of those important to you? We didn't need more seating or towing; we just got the X because the taller height made it a lot easier for my 84-year-old 6'4" 300lb father-in-law to get in. We must admit that it is a little more comfortable on road trips for us too, though not a lot.