RAID 5 and 5+1 will give you better speed than RAID 6 because it has to write to less drives. Also they key feature of 5+1 is that if a drive goes bad you have a hot spare to take it's place, meaning that you won't have to rebuild your array after getting another drive. You just buy a new one and that will take the place of the old hot spare. RAID 6 is the same as 5 but it uses an extra parity block (it takes up an extra drives worth of space) better for redundancy, but you will still have to take your array down and get another drive if there is a failure. I think the best option for availability and speed is 5+1.