portraitist vs portrait painter

portraitist and portrait painter both are nouns.

Word	Noun	Adjective	Verb	Adverb
portraitist	Yes	No	No	No
portrait painter	Yes	No	No	No

As nouns, portraitist and portrait painter are synonyms defined as:

Other synonyms of portraitist include limner, portrayer.

portraitist (noun)	portrait painter (noun)
a painter or drawer of portraits	a painter or drawer of portraits