Magnetic storage
Unlike optical storage, magnetic storage is re-writable.
Bus interface
While SCSI has a processor integrated into the controller, SATA makes greater use of the system processor to serve that function; the former is costlier.
Quality
Hard disk drives differ mainly in speed, storage capacity, size, redundancy (RAID), error correction, shock resistance.
Best and economical
Areal density doubling every two to four years since their invention. So, even 1TB is economical.
Optical storage
They are compact, but have the disadvantage of being over-writable a limited number of times.
Quality
Best and economical
Blu-Ray - where blue laser is used - is the rage. It is even more burdened than the DVD with copy-protection mechanisms like region code and DRM.
Flash memory
These use floating gate MOSFET transistors. They are fast and compact.
SD cards
Size
They come in 3 sizes: micro, mini and regular. A smaller card can be used in place of a larger card using an adapter.
Speed
Depending on transfer speed (in MBps), they are classified into various classes, with class 4 commonly used. Class 10 is recommended for watching movies etc..