RAID6 is unusable for many types of "always on" data, in particular databases backing high volume sites; the performance drops so much during a rebuild that for many uses it might as well be offline.
Then you use mirrors, you can apply a similar calculation there. Lower price per disk means you can put more disks in each mirror and compensate for lower per-disk reliability.
Obviously the type of redundancy used will depend on your specific application requirements, including performance.
Obviously the type of redundancy used will depend on your specific application requirements, including performance.