A disk failure is still a cost though in terms of restoring a backup, replacing the failed drive, waiting for an RMA/new drive to come, etc.. If the price delta isn't anything crazy I'd prefer to get something that I know is more reliable.
Earlier this year I had a Seagate fail within months of purchase, putting my NAS into degraded performance mode. So I quickly ordered a replacement drive (a WD Red) and started the RMA process for the Seagate. At some point during this process I removed and reinserted the Seagate, and... its been fine since. So now I have a WD cold spare in my closet for whichever drive fails next.
If you have a pool of 8tb drives and want to replace with a 7.9tb drive, you're out of luck.
Same here - created on each drive a partition slightly smaller than the HDD's capacity (using ZFS raidz1).