There are a plethora of options for storage nowadays. SSD's, Hard Drives, PCIE, SATA3, SAS, SCSI, NVME, M.2, there are all kind of options and things to consider, so what should you really be looking at? I'll run you through the various performance metrics to look at to find the right storage for you.
First thing I will talk about is capacity as that is what most people are familiar with. Capacity is just the amount of data that you can store. This is usally marked in GB(gigabytes) or TB(terabytes). A terabyte is 1000 gigabytes in the hard drive world. So if you see some SSD with a capacity of 500GB then that is half of the capacity of a 1TB hard drive. When looking at capacity you just need to know roughly on what magnitude you plan to be storing away. If you will be backing up all of the video for your media company then you will need much more storage than someone just using some spreadsheets and doing web browsing. Figure out roughly what your need is and make sure you can fit all of your data, either on a single drive or if you have to, you can buy multiple drives. You just have to check to make sure your system has enough slots for additional drives. If you dont have additional drives slots then you can purchase an external NAS or drive.
Next thing on the list is longevity. It's an unfortunate truth that none of our storage options last forever. All of them fail eventually. What you have to consider is how long you plan on using a certain system because likely your next system will have bigger and faster storage so if you are planning on upgrading every couple years then it is not entirely neccesary to buy a drive that has a 5 year warranty. If you are instead, trying to archive your family photos, then you will really want to check out what you can expect from a certain type of drive. Recently SSD's have shown that they can last much longer than hard drives if properly maintained, but if you have a lot of data then they might not be able to store everything you have. If you really do need extreme longevity and capacity then you may even consider a tape based system. Some of these have a warranty of 30 years and capacities of 15+ TB's.
Another thing to consider is type of failure. Hard drives fail mechanically and often the platters inside of them are actually very recoverable given that the read-write head hasn't scratched them or they have been otherwise physically damaged. This means, while the drive may be dead, the data still has a chance to be recovered. With SSD's it is a little more difficult because the flash chips can't be manually read as easily as a hard drive can. Currently we don't have any guaranteed methods to restore your data so a SSD with a good warranty might last longer but it might fail in such a way that you cannot recover your data and if you can it probably won't be cheap.
Finally we can talk about performance. As always you need to figure out what your use case will be. The storage performance for just backing up your files at the end of the day requires something a lot different than one that will make your daily computing experience responsive and enjoyable. The two main things to consider when looking at their performance is their read-write throughput and their IOPS. Read throughput is how much data from the hard drive that it can read per second and write throughput is how much it can write per second. In some scenarios you might need to read the data more often then you will need to write from it and vice versa, so consider that when you are choosing storage. If you are planning out a nightly backup system and need to backup 8TB of data and your drive has a write throughput of 200MB/s then you might not be able to finish the entire backup in a night and may need to look for an alternate solution. On the other hand IOPS is the number of individual operations a storage device can do in a second. This a high number on here will make your Windows load faster, games load faster and when you do a search on your computers files it will help that happen more quickly. This is a very important number for day to day computers rather than just large file storing systems.
Now that you have heard many of the various performance metrics to judge storage by I hope you can make a more informed decision.