The... um... mojo surrounding the word 'mystic' seems to have left with the 70's, but for me, the word totally conjures visions of The Oracle (like on the Hilarious House of Frightenstein) and other turban-laden swamis... practitioners of mind-melting mesmerization!
(Kudos if you read the last part with ALL of the exCITEment of SEVENties anNOUNcers!!)
Psion sucks for two reasons. 1: it's a made-up word that means nothing outside of D&D, and 2: it's homophonous with an unrelated real-life word that comes up a lot in fantasy. The first time someone suggested that I play a psion, I immediately wondered, "Scion of what?"
Psionicist is just too awkward to pronounce. Also, it's a made-up word that means nothing outside of D&D.
Mystic has many connotations, none of which have anything to do with psionics. Not a great fit.
Psychic seems perfect to me. It's a real word, it's easy to pronounce, and it clearly expresses the concept that it represents.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.