Another comparison of Cassandra and HBase based on an ad network company requirements:
We are an ad network. We need to store impressions and clicks. We were evaluating various big data (or nosql -what ever you may choose to call it) systems for our new project. We had used HBase for past 8 months in an experimental product and are satisfied with it but the hype about Cassandra was so much that we decided to give it a shot.
You should also check Cassandra, HBase, and PNUTS Compared