The X will also have the more stable framerate if you're into that. The S really is significantly weaker than the X and developers are already complaining that it's hard to run games on it. I'd say the S is in most cases a terrible proposition unless you really cannot afford more.
This is the first I've heard about devs having trouble with the XS S. I was kinda thinking I'd eventually buy an XS X for the 4K TV and maybe grab a cheaper S down the line for the other rooms in the house. Guess I need to do some research.