I'll put in my experience on this question first, "difference between hypnagogic and hypnopompic hallucinations".
I've been experiencing this as far back as I can recall, toddler age on, so about 30+yrs. As a child it was very intense and I didn't know what to make of it, my mother figured the house was possessed by demons, she was a bit over the top with religion. However here's what I've "seen", the hypnagogic stage was sometimes more frightening as the wallpaper would come to life, pictures would start-up and behave as TV's complete with sound, as a child this is a bit much! As I got older I found that I could control what was happening and what the outcome was, I also found that this gave me a completely new world in my dreams. I learned to completely control my dreams and become the 'creator' at about age 8, I would travel to other planets by flying AKA superman, I could make entire environments appear etc... I found that I would rather spend more time asleep than awake. who wouldn't?
As I got older I found that I could induce the dream state while awake, it was sketchy at first and required concentration, usually in the form of day time school. which by the way I always daydreamed during, this started around 2nd~3rd grade - I'd 'zone' out of class and disappear into my own reality, while still being aware of my surroundings and 'remotely controlling' myself so as not to appear too detached.
Now on the hypnopompic arena, that's entirely different. It can be a mix of waking dream into reality or waking and carrying in hallucinations that I don't recall in the dream state, I also think it's just hallucinations related to external visual field that my mind plays with. These are at first so real that it's impossible to tell the difference, it's the voice of reason in my head that says, "wait a moment, this can't be real" I then have a choice, I can try and interact with it at which point the hallucination fades or disappears or do nothing and watch it evolve into very fascinating things, and here's the key difference I've noticed, in the prior state of lucid dreaming while falling asleep the visuals and what not are just what ever I want them to be or have induced no real deep meaning or substance, more like and alternate reality. In the case of waking and dreaming, there is always a very deep meaning sense of feeling tied to it, I also find that it relates to something I've been trying to solve or work related with my design process. It's like seeing the overall picture all at once and getting a sense of where to focus and what and how to tie it all together to get this 'flash' where the proverbial light goes on.
It sounds very odd, and the experience is very hard to put into words as it's entirely for me a 'picture' based thing.