This isn’t a “better” vs. “worse” question. They differ in how they fail, how expensive they are, and what type of work they are suited for, and the failure differences are usually the ones you don’t realise until after you experience them.
Hard drives are going to give you the amount of space per pound; they also will retain your data when turned off. SSDs are going to provide you with the speed of access along with being resistant to shock. For most users, the most logical solution would be to use both.
The cost of spinning disks per terabyte cannot be beaten by today’s SSDs, so there are still plenty of applications for spinning disks, such as bulk storage, NAS boxes and storing files in archives. Spinning disks will also hold data when powered off for extended periods (a shelved drive will remain intact for years), but if you’ve got an SSD sitting unplugged it can slowly drain power over time. If your backup drive lives in a drawer or other area not always plugged in this could potentially matter to you.
In addition to this, when they do fail, most failures are mechanical with the data itself intact, making it possible to recover your files as opposed to a long shot.
With regard to speed, an SSD will be multiple times faster for everyday use and vastly faster for random access. This is the reason that having an SSD on your machine can give you the feeling of a completely different system. Since there are no moving components on an SSD there is also no noise, less heat generated, less energy required and less chance of damage if you drop or otherwise knock your machine (the largest benefit of using an SSD on a portable computer).
When it comes down to it, for the actual drive you work from, an SSD is hands down the best option by far. The only debate is about what you store on the other one.
The failure process of hard drives occurs over time and is audible. Bad sectors occur, transfer speeds begin to decrease until you hear the final click. Prior to this happening, you typically receive a number of warnings indicating that the drive is failing; and in many cases, even after the drive has failed completely, the data remains intact on the physical platters.
An SSD can fail instantly. It will normally shut down completely (silently). Usually it is the control board or controller. This is usually complete with little or no advance warning. The SSD works on Monday and then is undetected by Tuesday. SMART will indicate how many times the cells have been flashed (which in turn indicates how long they are expected to last) but will rarely indicate anything about how healthy your control board is. Adding TRIM to this as a feature means it will erase data that has been marked for deletion, almost immediately. So overall, an SSD is less predictable and also less forgiving.
System SSD, and working files. Hard disk for video/movies/music etc., and backups. Using this configuration allows you to take advantage of each type of device’s best features, and will cost less than having a full SSD.
Both types of drives fail so what you need to focus on is backing up, and one of them can fail without any advance warning. If a drive has already failed, then after a free 48-hour diagnostic, recovery can cost from £300 +VAT for a single drive.
Price per terabyte: nothing else can be compared in terms of mass storage. Hard drives are good at holding data powered down so they are suitable for archive or backup purposes; and because your data is usually still stored physically on the platter(s), there is a good chance of recovering them after failure.
The moving parts (which wear out) cannot survive being struck while rotating, take longer to access than an SSD, and are sensitive to both temperature and vibration. Each hard drive has a service life and will fail eventually.
While physical failure does occur less frequently with SSDs than with hard drives, predictability is much lower. Most controller failures that result in catastrophic failure happen without warning with an SSD. Hard drives typically give audible warnings before they fail, and this is a real advantage if you act upon it.
Generally, use a hard disk for storage: cost per terabyte matters most for backup space and hard disks store data better when left unpowered for long periods. Use an SSD as the disk you work from.
The majority of the time it will be a hard drive, and a mechanical failure will leave all the data still stored on the platter. For an SSD, TRIM actually deletes the deleted file within a few minutes and if the flash memory itself has deteriorated then there may be nothing to read.