The problem is, such distinctions are only meaningful to people who care about them.
From a practical point of view, "Trekkie" tends to be more useful in everyday conversations. If you're at a PTA meeting or neighborhood barbecue and you try to explain that, no, seriously, you're a "Trekker," not a Trekkie, people are just going to look at you funny.
Which, ironically, may be the opposite of what you intended . . . .
Yeah, they're going to think that anybody who's given that much thought to the subject has a couple of screws loose, and what's more, they may be right!![]()
Plus, proudly identifying yourself as a Trekkie suggest that you have a sense of humor about it--and aren't at all self-conscious or defensive about being a Trek fan.