When trying to evaluate NoSQL databases, its usually better to try them out. While trying them out, its better to use them with multiple node configurations instead of running single node. Such as clusters in Riak or Replica-set in mongodb maybe even a sharded setup. On our project we evaluated a 10 node Riak cluster so that we could experiment with N, R and W values and decide which values where optimal for us. In Riak here is what N, R and W mean.
N = Number of Riak nodes to which data will be replicated
R = Number of Riak nodes which have to return results for the read to be considered successful
W = Number of Riak nodes which have to return a write success before the write is considered successful