In a RAID6 with 8 drives, you can allow one disk to go offline and remain able to write, or two disks offline and remain able to still read 100% of data. You lose 2 of 8 = 25% capacity on redundancy.

With a mirror of 2 disks, if one disk dies, you can still read; if two disks die, you're toast. And you lose 1 of 2 = 50% of capacity on redundancy.

A quite different balance.