They are both equally good IMO. If it is just to check drive status, crystal disk is always free, easy to use, and less junk.
We have been getting HDSentinel on ALL off our clients servers for reporting and emailing of failing drives and man has it come in handy.
Now USB/SATA/PATA drives Crystal Disk works great but it can not read SAS/SCSI drives unlike HDSentinel (which is why we use that). Crystal Disk also can't read though all of the RAID cards, but then there has been on RAID card (a fairly new and high end RAID card) that HDS can't read though either but otherwise works great.
So it really depends. If it is personal. Just Crystal Disk. If it is on a server and you need email reporting or dealing with SAS/SCSI drives go HDS