I like to keep the canonical neat so it looks better in the serps so as to encourage future users to click on it (people like clean, readable URLs), so I try not to use unnecessarily complex URLs. I have had very good luck in canonicalizing messy URLs with decent authority to new, completely non-ranking but clean, URLs and not only keeping but growing page authority.
And I have a fairly large site which is crawled regularly so I do value consistency--whichever page is set as the canonical will eventually become the ranking page, so current rank is not the biggest issue to me. In the long run, consistency will make your life (and the life of anyone who follows you in your present position) easier. (That being said, it wouldn't hurt anything if you prefer to use what is currently ranking.)
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