I take exception to those who have said or implied that it's better not to use what they consider "big words."
This incident aside, some people do have an extensive vocabulary and should never be mocked for it. I agree with the poster who has said that "antithesis" is not particularly high-falutin.
One caveat would be to know your audience, because of course the most important aspect of speaking is that people will understand you.
~Who in the world am I? Ah, that's the great puzzle~ Lewis Carroll, Alice in Wonderland