I think the way each of them dances with various people reflect their contrasting natures.
Ennis didn't really know how to dance before meeting Cassie. The first dance was simply a suggestion of a dance -- doing the motions while Cassie "led" him on. The second instance showed him a bit less awkward -- but still just going through the motions. When he excused himself to Alma Jr. in the second dance, you could tell that he did not seem to relish Cassie's "invitation". At best, he was awkward, and knew it. He did not enjoy it.
We see Jack dance more often -- with more fluidity and ease. And yet, what struck me is the way he was able to hide his own feelings behind the fluidity and grace.
For me, the most beautifully done and revealing dance scene is Jack's first dance with Lureen. At the start, we see slight flirtation in the way Jack taps Lureens fingers to the beat of the music, the way he makes eye contact, and the casual conversation that they seemed to share while dancing, and his seeming mastery of the situation (Lureen seemed smitten, and he played it to the hilt!) All bespeaks ease and enjoyment, until Jack's face is hidden from Lureen's and we see Jack's soul:
