WD MyCloud

I am intending to buy a WD MyCloud 3 or 4TB and transfer all files from my iMac Hard Drive to it. I will also back this up with an existing hard drive which I have.

Is it worth the extra cost to go for the 4TB Mirrored version?

No, there is no extra cost for mirrored version of My Cloud. Only the RAID configuration defines the RAID behavior either it would be mirrored or stripped.