syntagm vs word string

syntagm and word string both are nouns.

Word	Noun	Adjective	Verb	Adverb
syntagm	Yes	No	No	No
word string	Yes	No	No	No

As nouns, word string is a hypernym of syntagm; that is, word string is a word with a broader meaning than syntagm:

syntagm: a syntactic string of words that forms a part of some larger syntactic unit
word string: a linear sequence of words as spoken or written

Other hypernyms of syntagm include linguistic string, string of words.

syntagm (noun)	word string (noun)
a syntactic string of words that forms a part of some larger syntactic unit	a linear sequence of words as spoken or written