I'm in a loaner from my SC, a Model S 75, while my X90D is spending the weekend there (nothing to tell. Just a tire replacement; they didn't have one on hand so loaned me the S while they wait for it to come in). Until a few hours ago, I had never actually driven an S model - really one of the most beautiful cars on the road, IMHO.
The drive is very nice and handles exceptionally well. And, of course, it's super quick. Comfortable and seems slightly quieter than the X. I could be wrong, but the kWh usage seems noticeably better, too. The general driving features/components are seemingly identical to my car. I did notice that I'm getting a lot more pointing and staring at the S as I sit at lights or roll through a parking lot. People remain awed and amazed at the beauty and concept.
But, I am so glad I bought the X.
Primarily, I like the higher seating profile for access and driving. I'm no giant, but at 6'2"/200, I'm having a hard time folding myself into the cockpit of the S. It's quite low to the ground. Even with the seat way down and back and steering wheel forward I have to really crank my head/neck/shoulders to the left as I lower myself down with my left leg and sling my rear and right leg in first and kind of "plop" into the seat; and then bend my head in. This all must be done in a coordinated, fluid motion. Getting out, I throw both legs out first, turn my entire body counter-clockwise, duck my head under the frame and lurch my way out and stand up. I've been getting in and out of cars for 4 decades and have a pretty good knack for the process, and this is noticeable. Also, that low profile while driving is less pleasant to me.
Conversely, I've gotten so used to the space and openness of the X interior. The S feels much more compact and enveloping on the inside.
Just one man's opinion/observation.
If you're on the fence, get an X. ;-)