Defining stylistics is a difficult problem. An example of the relationship of stylistics and syntax is the sentences of Marcel Proust. His style is characterized of very long sentences. However his long sentences are rather graphic sentences than syntactic sentences. Even so his syntactic sentences, often included in his graphic sentences, are long hypotactic sentences expressing the narrators associative "search of lost time".
A very short answer: stylistic concerns the speech acts, the saussurean 'parole' of the speaking individuals. Syntax concerns the language rules (of a single language as well as the general rules that can be found in all languages of the world ; i.e. the saussurean 'langue')
It is the relation to norm. Syntax is part of grammar, and grammar is by definition restrictive. It is a set of rules or principles that tells apart the well-formed expressions (of English, Chinese, etc. ) from the ill-formed ones. The grammatical capacity can be called intrinsic norm of the language, although this terminology is not popular. The object of stylistics is a matter of convention. It makes sense to assume that stylistics primarily deals with outer norms that make up a prescribed form of the language, but can also deal with related issues, for instance, with social stratification of language idioms or profiles of individual speakers. Well-formed syntactic expressions can be infelicitous in many contexts and even considered stigmatized from the viewpoint of stylistics.