HANA is developed to be a very fast general-purpose database and application platform, while BW is an application that operates as a toolkit for building and Keepingup data warehouses. Determining which product logic for someones use case is a huge topic, but I say an overview of the decision making technique in this blog on how the option with SAP BW and HANA depends on someones data management requirements.
Despite addressing two different needs, SAP BW can benefit from SAP HANA, and vice versa. SAP BW requires a solid application platform (recently NetWeaver ABAP fills the role) and a fast database, especially for analytic workloads. HANA fits the bill here. Meanwhile, businesses creating data marts and data-centric applications on HANA will eventually begin to sense the pain of working a large and different data environment. BW’s data warehousing competences can help here.
The “SAP BW on HANA (Training)” version of BW is already becoming quite associate with HANA, dragging HANA’s capabilities to provide features that are unavailable when BW is running on other databases. I believe that over time the multiple platforms will extend to become more connected. The article By what means SAP BW and HANA will merge over time — and why it matters walks through some of the possible ways the two applications will come nearby together.