Hbase vs Cassandra: Which is better for a timeseries data storage?
Advantages of Cassandra:Cassandra generally shows better performance (though both are excellent).Cassandra is substantially easier to setup and manage from an operational stand point (though there are tools that will help either way).
Advantages of HBase:Native to the hadoop ecosystem
HBase will require you installing hadoop anyway, and you get a nice two-for-one. To use Cassandra you will probably need to go to use DataStax Enterprise, a commercial, non-open source product, OR investigate using Spark for your analytics work which has an open-source connector with Cassandra.
Chocolate or Vanilla ice cream - which is better?
I would suggest that you would be the best decision maker. Set up development environments for each option, and this will tell you much more about operational and tuning issues than, I think, anyone else might be able to give you. :)