I have to agree with the other comments that you seem to have prematurely narrowed your options. The requirements that you mention are pretty broad and don't really pick out Riak or Cassandra in particular.

You may want to ask, What are your requirements for data consistency? Will you need to build reliable abstractions on top of your data store? If so, you may want to look at other options, such as FoundationDB.

Stephen Pimentel, foundationdb.com