When you use blkdiscard, the SSD (assuming a SATA SSD) will receive an
ATA TRIM command, whereas hdparm --security-erase will issue an ATA
SECURITY ERASE UNIT command.
Personally, unless I want to keep some of the data on the drive, I use
hdparm --security-erase, as I feel that this is most likely to reset the
drive to its as-new state