personally I'd go with Jennifer Lopez over Ciara. As a dancer, she's showcased more throughout her career than Ciara has. A plethora of styles, tempos, and concepts in the world of dance. Ciara does what she does well, but it's pretty limited.
Jennifer Lopez does not possess the flexibility of Ciara though.
Lol I don't think flexibility is what would make her a better dance, if that was the case Britney during her younger years would be considered better than everyone. she was splitting, flipping and doing all kinds of gymnastics stuff.