Recently Prince Charles used the word 'Hitlery,' in the sense of "possessing some properties of Hitler."
Is there any difference between the suffixes -ish and -y ?
Michael Quinion, Ologies and Isms: A Dictionary of Word Beginnings and Endings (Oxford, 2002) offers extensive entries for -ish and -y. The entry for -ish as a suffix forming adjectives is further subdivided into adjectives formed from nouns and adjectives formed from other adjectives:
One set from nouns is of adjectives for a member of a nation. A second set from nouns is of adjectives that indicate its qualities or characteristics.... May are formed from proper names, often for a single use (Ayckbournish, Hockneyish, Thurberish). Though these examples are neutral in tone, the great majority are derogatory.... Examples formed from other adjectives suggest some quality that is roughly or somewhat like that of the adjective (coldish, dullish, loudish, moreish, oldish, sweetish, tallish, weakish).
The entry for -y is even more extensive, stretching across three separate entries. The first of these is potentially relevant to the OP's question. Here are excerpts from it:
-y Also -ey Full of; having the quality of; inclined to; apt to. [Old English -ig, of Germanic origin.]
Adjectives in this ending divide broadly into three group. In one they straightforwardly denote the quality of the nouns from which they derive.... A second group are to some extent dismissive or disparaging, often with a figurative or indirect association, such as beery, boozy, dreamy, mousy, tinny. A third set indicates a close attachment or mild addiction to something, as in booky, doggy, horsy.
The ending is active and is often used to create informal terms which may be mildly negative in tone, frequently in a an attempt to communicate some quality that might be hard otherwise to describe briefly: bacony, dancey, designery, Internetty, jargony, plasticky, tabloidy.
To sum up, Quinion appears to be saying that -ish creates adjectives that suggest a rough equivalence to the originating noun, often with a derogatory edge, and that -y creates adjectives that express the quality of or an inclination toward the originating noun, but that may be neutral, disparaging, closely associative, or mildly negative.
I can't for the life of me tell whether Hitlery is more disparaging than Hitlerish or less disparaging, and I doubt that Prince Charles can either.