The other day I was building a new lun and had the option of raid 5 or raid 6… Hmm.. 6 must be better because it is newer? Not so much.. Is it better? Depends on your criteria of better? More resilient to outages, absolutely. Gives you more storage.. Nope.
So when should you use raid 6? Well with some arrays you have no choice and that is fine, they typically have worked out some of raid 6’s shortcoming, but if you are sitting there with a decision that you normally would have had no problem choosing raid 5, ask your self how quickly are failed drives swapped out? If your answer is less than a day, then you probably will be fine with raid 5.. If your answer is in a term of days, or worse weeks, raid 6 will give you a better chance of keeping your data.
Now please note that for NL SAS and SATA raid 6 is the recommended due to the rebuild time associated, so please take that into consideration, considering the statistics that two drives sitting next to each other have a slightly higher probability of failing than a totally random drive out of your array.