Microsoft recommendation is to use RAID 10 both for logs and data/index files. When cost is an issue you could allocate data and index files on RAID 5 or equivalent; however, keep logs on RAID 10.
RAID 10 offers better availability than RAID 5 and better performance especially for write-intensive applications. Some of our customers have reported up to 50% adverse impact on write intensive work load when moving to RAID 5. The impact on performance of RAID 5 is dependent on the hardware vendor’s implementation of RAID 5 and the characteristics of your particular work load. You should not assume that you will get a 50% adverse impact – it could be higher or lower.
http://blogs.msdn.com/b/sqlcat/archive/2005/10/11/479887.aspx
