"CIDER (formerly nrepl.el) is the Clojure IDE and REPL for Emacs, built on top of nREPL, the Clojure networked REPL server."
That and the availability of the packages on melpa:
cider 20140318.... available Clojure Integrated Development Environment and REPL
And the fact that technomancy's git repository for nrepl.el hasn't been touched in 2 years leads me to believe that cider is the better nREPL choice ;) Be happy to hear that nrepl.el is still maintained and is better in some way than cider, but I don't see cider as being that heavy (at least as compared to SLIME).
