Database for a startup
NoSQL and RDBMS are totally different animals.  You should pick the one which suits your data and how you intend to record and access it.

Don't pick your style of database system on the grounds of performance.  Performance can be managed in a range.  If HBase isn't performant at your start up volumes, then maybe you'll just have to work harder to tune it, or throw more money at hardware, or just live with slightly lower performance until your volumes catch up to HBase's sweet spot.