I understand that both kingdoms represent primitive eukaryotic organisms, and both contain both unicellular and multicellular organisms.
What are the points that distinguish one kingdom from the other?
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First, just a correction: it's "Protista" and "Protoctista", not "Protistae" and "Protoctistae".
They mean the same thing. Those are just different names proposed by different taxonomists for describing the same group of organisms.
However, as it is common in taxonomy and nomenclature in general, things sometimes get a bit more complicated: some biologists, like Lynn Margulis, don't treat those two terms as synonyms. Particularly, I don't use her nomenclature.