Personally, my preference for zookeeper comes from the API. To me, the ZK API and docs were far more understandable than the etcd ones. The etcd api docs appear to be a collection of examples, not reference docs. It does a poor job of explaining the possible operations and what various options will do, particularly in what combinations of options are allowed.

In fact, I had to resort to running test queries against a running etcd server just to work out the proper semantics of some of the arguments.