It sounds like this external drive will be to store important & unique data. If this is the case, then you will also need to have a backup plan in place for your external drive in addition to the main system drive on your Mac. Also RAID is not a backup so don't get the Thunderbay drive over the Mercury Elite thinking the RAID is better because it provides a "backup" since it includes multiple drives. It may be possible to configure the Thunderbay unit so that it stores your data with half the drives and the other half of the drives is grouped together for your actual backup drives. This is not ideal if it is your only backup since if this enclosure fails, then you lose access to both your large amount of data (photos & music) and your backups.
If you have multiple computers, then instead of directly attached storage you may want to consider a network storage option (NAS) such as the ones provided by Synology.
Yes, things start getting complex when dealing with larger amounts of data. Take your time to read up & learn about the options and terminology. Now is the time to figure out the best option for the next five years or so and how your needs may change or grow over that time.