Cassandra is by most accounts a pain to setup and administor. Mongo has good docs.
Administration is pretty hands off for most tasks. For instance you want three copies of each piece of data? Set the replication factor to 3. Other parts are difficult to "get" if you don't have an understanding of the code, my best advice is to make sure you approach Cassandra from a developer perspective. You can't treat it like a black box quite yet.