In Acts 24 the Lord gives us a graphic contrast of speech in the orations of Tertullus and Paul before Felix, the former being recruited by the Jewish elders as a prosecutor.
One can't help but notice Tertullus' florid speech. He uses big words (seven are unique in scripture) to flatter Felix and misrepresent Paul, showing himself a liar on both accounts--albeit an eloquent one!
Paul uses plain speech to defend himself, in keeping with his statements in 1 Cor. 2:1 ("...I..came not with excellency of speech...") and 2 Cor. 3:12 ("...we use great plainness of speech"), and it truly is a "demonstration of the Spirit and of power" (1 Cor. 2:4).
Nothing wrong with eloquence, as long as it's used for the glory of God. Apollos provides a fine example of that for us in Acts 18:24-28. He's eloquent, but also a sincere truthseeker who humbly receives correction from Aquila and Priscilla.
Moral: big words are not what win the day, but a big God working through men with big hearts!