I am a Trekker
because I am a serious Star Trek fan
A Trekker is also "one who treks". This fits me, and I would love to be among those who trek to the stars, I.E. a Star Trekker
"Trekkie" is a what Star Trek freaks are referred to by outsiders, or perhaps self-professed ST freaks.
See: the "Trekkie" movies.
"Trekkie" is the Star Trek version of "Hippie".