Which of the two terms should I use to indicate skilled TeX users in "official" contexts, for example, a TUGBoat article?
Here also TeXies is used, but I think this one is informal.
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Quote from TeXbook, chapter 1: The Name of the Game
English words like 'technology' stem from a Greek root beginning with the letters τϵχ...; and this same Greek word means art as well as technology. Hence the name TEX, which is an uppercase form of τϵχ.
Insiders pronounce the χ of TEX as a Greek chi, not as an 'x', so that TEX rhymes with the word blecchhh. It’s the 'ch' sound in Scottish words like loch or German words like ach; it's a Spanish 'j' and a Russian 'kh'. When you say it correctly to your computer, the terminal may become slightly moist.
When you write "TeXpert", you might be tempted to pronounce the "X" not as a Greek chi but as an 'x'.
Besides this the term "TeXpert" may indicate that someone is savvy regarding the technology related to TeX. But the term does not indicate that much that there is also an aspect related to arts.
Thus the answer to exercise 1.1 of the TeXbook
After you have mastered the material in this book, what will you be: a TeXpert, or a TeXnician?
A TeXnician (underpaid); sometimes also called a TeXacker.
An answer to your question could be:
A TeXnician according to the diction of Donald E. Knuth is somebody who has mastered the material of the TeXbook.
Knuth does associate having mastered the material of the TeXbook with the term "TeXnician".
Knuth does not associate having mastered the material of the TeXbook with the term "TeXpert".
Thus with a "TeXnician" doubts about her/him having mastered the material of the TeXbook are excluded, while with "TeXperts" these doubts are not excluded.