What is the difference formation wise between the x and s receiver positions?
The ball being placed on the left or right hash creates a wide-side and a narrow side of the field. The X-receiver always lines up on the narrow side. Given the more limited space to operate in, there seems to be a premium on getting big, physical receivers to play the X. Since the defense stacks itself towards the wide-side of the field, there also a lot of big play potential for the X if he can beat man coverage.
The S-receiver is the widest on the wider side of the field with the slot/flanker and/or tight end almost always on the same side of the field as the S. While you’d love for the S-receiver to be 6’4/200, you have to balance that against the need for the S to be fast (to stretch the defense and create opportunities underneath for the slot or tight end) and to be able to excel at a variety of routes that are being run in combination with other receivers stacked on that side of the field. So you want the S to be equal parts big, fast and technically sound.
When a receiver comes into the program they are trained in one role. They aren’t interchangeable. Once they gain more experience, a receiver with a good football IQ can develop the ability to play multiple receiver positions.