A section from Toshiba - trends in technologies (sorry, can't post links yet)
"SSDs and HDDs are both major storage devices, each with their respective strengths. Although HDDs are
widely used as storage devices that have good balance with respect to capacity, performance, and reliability, they have data access speed limitations due to their mechanical structure. By contrast, although the access performance of SSDs exceeds that of HDDs by a two-digit figure, HDDs are still less expensive in unit price per gigabyte. As demand for data storage grows in terms of capacity and performance, both drives will coexist by complementing each other."