RDF
As of version 3.3, these two properties aren’t related/equivalent in the RDF. Here are their definitions (in Turtle RDF):
schema:author a rdf:Property ;
rdfs:label "author" ;
schema:domainIncludes schema:CreativeWork,
schema:Rating ;
schema:rangeIncludes schema:Organization,
schema:Person ;
rdfs:comment "The author of this content or rating. Please note that author is special in that HTML 5 provides a special mechanism for indicating authorship via the rel tag. That is equivalent to this and may be used interchangeably." .
schema:creator a rdf:Property ;
rdfs:label "creator" ;
schema:domainIncludes schema:CreativeWork,
schema:UserComments ;
schema:rangeIncludes schema:Organization,
schema:Person ;
rdfs:comment "The creator/author of this CreativeWork. This is the same as the Author property for CreativeWork." .
As can be seen, they don’t refer to each other on the RDF-level. The sentence ("This is the same as the Author property for CreativeWork.") in the human-readable definition of creator
is the only place that says that these two properties are equivalent.
Differences
While both properties can be used for CreativeWork
items, only author
can be used for Rating
items, and only creator
can be used for UserComments
items.
Which to use?
As UserComments
shouldn’t be used anymore, the author
property would be the more useful one if you only ever want to use one of the two properties.
However, as the CreativeWork
type is not only for works that have, in the natural langauge sense, an author (documents etc.), but also for works that have a creator (e.g., sculptures, for which there is the Sculpture
type), authors of the structured data might want to prefer the creator
property in these cases.
Anyway, in Microdata and RDFa, it’s easy to use both:
<span itemprop="author creator"></span>
<span property="author creator"></span>
(You can use both in JSON-LD, too, of course, but it’s not that simple.)
So if you care about the RDF, and until Schema.org makes these two properties equivalent on the RDF-level (if ever), you might want go this way if you think that an interested consumer only supports one of these two properties.