Rhetoric Vs. Dialectic

Rhetoric
The art of effective or persuasive speaking or writing, especially the use of figures of speech and other compositional techniques.

Language designed to have a persuasive or impressive effect on its audience, but often regarded as lacking in sincerity or meaningful content.

Synonyms: oratory, eloquence, command of language, way with words

"a form of rhetoric"


Dialectic
The art of investigating or discussing the truth of opinions.

Synonyms: discussion, debate, dialogue, logical argument, reasoning, argumentation.

"Science has of course contributed to this dialectic"


Summary
Use rhetoric when debating a person caught up in passion. When logic will fall on deaf ears. When people are overly emotional. On Twitter.  On Facebook.

Use dialectic in formal debates, blog posts, and professionally published articles.  Do not use dialectic in an argument with an overly emotional person.

Use a little of both when teaching, lecturing, and giving speeches.