What I notice with sx, so, and sp is that each has a certain "flavor". Sx tends to have some edge, so has more softness, and I agree that sp has utility - I also tend to see out of sp more "classic" styles. But I think it's also in how
clothes are worn, not just what they are.
sx/sp 9 ISTP brother tends to dress preppy/casual/sleek. He's slender and muscular, and is usually in a close-fitting v-neck t-shirt, sometimes a button-down over that, classic khakis and boat shoes. Corduroys in the winter.
sx/so 6? INFP? that I know at work wears brightly-colored summer dresses most of the time, with lots of jewelry and funky shoes, light makeup.
sx/so 6 ENFJ that I know at work wears tight-fitting yoga pants or close-cut jeans and tight-fitting feminine cut t-shirt tops, sometimes smoky eye makeup.
sx/so 7 ENFP? aunt is a huge fashionista and wears all sorts of Prada and Dior and tight dresses and slim pants and animal prints and crazy heels, makeup.
sx/sp? 9? ESFP? friend from school has tons of colorful tattoos and wears tight-fitting jeans, tops, and dresses (she's a bartender), with dark eyeliner and bold lips.
It seems like sx/so style is more flashy and sx/sp is more dark/edgy...