Based on my experience (which does not include a formal diagnosis, but I'm confident about this), PLF is more of a constant "acute" dizziness... Like really a sensation of spinning, or the floor/bed/chair moving, sometimes violently. PCS is much more mild, like a gentle rocking, or an occasional "sharp" rocking feeling, but mostly more of a vague disorientation, often in a physical space, rather than an acute dizziness.

My background includes a lot of physical movement stuff (gymnastics, climbing, skating, martial arts, etc) so I'm more aware than most people when I "switch" from vestibular input to tactile and visual input to determine where I am and how I'm moving (we all do this sometimes, but most people are not aware of it; usually vestibular input is in charge, but sometimes gets help from visual and tactile input). My experience made it VERY clear to me that my vestibular input was broken. It was sending "noise" instead of "signal" to my brain.

I've got a strong stomach, so I never really got "sea sick", except immediately post-concussion.

Two stories that may help you figure out if it's more likely PCS or PLF...

1- I was sitting in an office and someone was showing me a google-map. I was watching the screen when they quickly zoomed in and I nearly fell out of the chair. I had to close my eyes, grab the sides of the chair, feel that my feet where firmly flat on the floor without pushing too hard, and push myself back into the chair.

2- I had just gotten off a bus and I was walking (like a drunkard!) along a row of stopped buses. The buses were taking up about half my field-of-view. When the buses pulled away, I nearly fell over. I had to look at my feet, partially close my eyes, hunch a little bit to drop my centre of gravity and focus on the feeling of weight in my feet; shutting out the visual input and (consciously) focusing on tactile feedback (the feeling of weight in my feet; left/right/front/back).

Those are, for me, examples of using my visual input (and then tactile input) to compensate for vestibular input that was not working. When the visual input changed from a steady point of reference to something that was moving, my brain automatically tried to hold me "steady" against those moving points of reference: not gonna work! A steady reference is required to hold oneself steady!

The "switch" happens for most people automatically, and contributes heavily to cognitive fatigue... The brain just isn't meant to process visual and tactile feedback that way, at least not all the time. PCS, in my experience, doesn't result in that compensation resulting from vestibular input being "broken".