I'm seeing this suffix everywhere lately. Of course, there are a number of -esques that are commonly used (i.e. Kafkaesque), but is there some sort of rule for determining who (or what) gets assigned which suffix? For instance, was Orwellian chosen simply because someone decided it sounded better than "Orwellesque"? What about "Kafkanian"? It all seems very willy-nilly.
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From what I could infer based on the Wikitionary entries, -esque is more likely to be appended to proper nouns than -ian.
Also there appear to be nuances in meaning though they seem to have been dulled by conflicting contemporary usage. -ian can be used to mean "related to" in addition to "like", e.g. Machiavellian could relate to the principles of government as expounded by Machiavelli or something befitting Machiavelli .
-esque is generally used only in the sense of being like something or befitting something, e.g. "Machiavellesque" could describe something befitting Machiavelli, not necessarily related to his principles (although I am inclined to leave this point open for discussion).
Based on these arguments, I conclude that -ian can be used in a much broader context than -esque even when both are appended to proper nouns.