This article gives a hint:
A single NAND SSD can achieve 10-30K random I/O operations per second (IOPS), while a single SAS disk can achieve perhaps 250 IOPs. For some database operations, such as random reads or writes (e.g. using an index) the SSD may outperform the disk by 3-4 orders of magnitude.
It also says that there is a point below which buying Flash drives turns out cheaper than buying hard disks – agreed that smaller capacity requirements aren’t prevalent – they can be used as a intermediary cache between main memory and hard disk thus adding to the performance contributed by the chain.
NAND flash is fundamentally a semiconductor device and benefits from the increased density due to Moore’s Law. Hard disks also benefit from exponential increases in density for storing data, so there is no risk that NAND will overtake hard disks. However, the disruptive aspect of NAND flash is that the cross-over point changes over time – in tune with Moore’s Law. Today, the cross-over point is ~16GB; in two years from now it will be ~32GB, and in four years, 64GB.
…… So for users with a large appetite for media, hard disks will still make sense for a long time to come. … But the general trend seems to be that the market for storage will split into several pieces: cost focused, performance focused, and capacity focused. Right now it looks like the first two may be dominated by NAND flash, rather than hard disks.