There are a lot of plugins available for leiningen, which add clojure centric functionality like searching on clojars etc. gradle on the other comes with a lot of pre-made plugins for other languages and tasks, scala, groovy, java, war, jetty, etc. And it picks up a lot drive lately.
So it basically depends on what you want to do, I guess. If you have to be flexible and integrate with other languages or interface with some legacy build system, gradle is certainly worth a look. If you "just" use clojure, leiningen gets you up and running in no time.
Sincerely
Meikel