I'm more inclined to think the instinctual stack doesn't have such neat "roles".
I feel a greater focus on sp matters, so that when out of order I am preoccupied with getting them back on track. This is not health or security stuff, but more 4ish sp stuff. It's more about independence, little luxuries & indulgences, and experiencing/projecting elegance.
For what it's worth, I relate to ALL instincts when interpreted through the 4 core fix. That's why sometimes I think it's just "being a 4".
But the sx-dom 4 just sounds like a more volatile, extreme kind of person, for lack of better words. I imagine Johnny Depp trashing a hotel room or Marlon Brando getting expelled from HS for riding his motorcylce through the halls. I'm much more withdrawn & not exhibitionistic like that - or I much more passively seek to assert an image & stand out (ie. physical image).
There's some description of sp/sx types seeming to be underneath layers of bubble wrap, but writing/saying things that leave an effect like stabbing people in the chest. That just sounded much more like me, whereas sx-dom seem more "receptive".
But Sanjuro & Amargith bring up interesting points too. Based on all this, I might say I am sx/sp, but taking into account other stuff & the impression I leave on most people, sp/sx might be a better fit still.