RAID5 can withstand only a single device failure.
A mirrored metadevice can withstand multiple device failures in some cases (for example, if the multiple failed devices are all on the same submirror). A RAID5 metadevice can only withstand a single device failure. Striped and concatenated metadevices cannot withstand any device failures.
When a device fails in a RAID5 metadevice, read performance suffers because multiple I/O operations are required to regenerate the data from the data and parity on the existing drives. Mirrored metadevices do not suffer the same degradation in performance when a device fails.
In a RAID5 metadevice, parity must be calculated and both data and parity must be stored for each write operation. Because of the multiple I/O operations required to do this, RAID5 write performance is generally reduced. In mirrored metadevices, the data must be written to multiple mirrors, but mirrored performance in write-intensive applications is still much better than in RAID5 metadevices.
RAID5 involves a lower hardware cost than mirroring.
RAID5 metadevices have a lower hardware cost than mirroring. Mirroring requires twice the disk storage (for a two-way mirror). In a RAID5 metadevice, the amount required to store the parity is: 1/#-disks.
RAID5 can't be used for existing file systems.
You can't encapsulate an existing file system in a RAID5 metadevice (you must backup and restore).