I suspect that there truly is a worthwhile distinction lurking in here somewhere.

Good analysis, Father Steve.

It could be that the language inclines to the "ist" ending when describing a follower of a historical figure like Marx or Lenin or Calvin who urged profound changes in the organization of society or religion, whilst inclining towards the "ian" ending to describe those who "accept" intellectually, but do not "follow" [implying commitment or devotion], the world-view or the hypothesis of a pioneering scientist, like Newton [Newtonian], or Freud [Freudian], or a pioneering thinker or philosopher, like Aristotle [Aristotelean].

Of course, this doesn't explain why a follower of Christ is a "Christian" rather than a "Christist".* [Please see "Footnote" - separate post.]

The awkwardness of the "Christist" alternative may be revealing. It may be that exceptions from the 'rule' occur only when needed to soften or facilitate pronunciation.

Further, it may be that the "ite" ending is preferred over "ian" for prominent creatives, like Barzune, whose literary or artistic insights do not shake the foundations of science or society, but merely lead the way in establishing new perspectives or styles or fashions or trends.

Interesting that Marx gives us both "Marxian" and "Markist" indicating that his world-view was sufficiently epochal to be studied or 'followed' or both.