SSD versus Enterprise SAS and SATA disks
 

The superiority of the Intel SSD drives is simply astonishing. Even eight of the fastest SAS drives are not enough to keep up with one (!) SLC SSD drive. The high seek time of our Western Digital (8.9 ms) also kills performance: 16 drives are slower than four 15000RPM SAS drives. The eight drive score of the Western Digital setup gives us an idea of how many SATA drives you need. It will take about 26-30 SATA drives to get the performance of eight SAS drives… and it will probably take about 40 SATA drives to beat one SLC SSD disk! The more your applications read and/or write randomly, the worse the "get a lot of cheap SATA spindles" plan becomes.

http://it.anandtech.com/IT/showdoc.aspx?i=3532&p=1