RAID-5 uses up 1 disk worth for striping, so net space in an 8-drive array is 7-drives worth (about 27TB using 4TB drives). The problem with RAID-5 is that you are 2 disks away from failure and rebuilds often kill the disks.
RAID-6 uses 2 disks worth for striping, so net space in an 8-drive array is 6-drives worth (about 23TB using 4TB drives). Is able to survive a double-disk failure before data loss. Still has some of the same issues as RAID-5.
I use Greyhole for media and document storage. It handles disks of unequal size (currently running one 3TB and two 1.5TB drives), and you can choose the level of redundancy you need. In my case, movies, TV shows, etc. get a single copy (one file exists on one drive), while documents and photos get two copies (one file exists on two drives). If a drive goes bad, you only lose the files on that drive...and only for the files for which you selected no redundancy. With redundancy, extra file copies are recreated on the remaining drives from the surviving copies; this process is most likely less stressful on the disk set than a RAID rebuild.
My movies, TV shows, and music are backed up to BD-R, stored in a binder at work. They hold ~20GB each, as I'm using dvdisaster to guard against media errors. When a 2TB drive failed, I brought the backup (currently about 190 discs) home and restored the files that had gone missing. Backup and restore are managed by scripts, with information about what files are on what discs held in a MySQL database that gets periodically backed up off-site as well. The initial backup took several months (on and off) to finish, and the last time I needed to restore, it took about a week, but now I just burn a disc when I have about enough new data to fill one. Burning and verifying takes a few hours, but it's something you can start and walk away.