I thought you had that decided already?
I think you are confusing HDD with SSD.
HDD = Hard Disk Drive
It uses magnetic rings to store data (have you ever broken a floppy disk? Like that, but bigger, and data storage is much more refined).
SSD = Solid State Drive
It uses flash memory to store data (have you ever broken a USB memory stick? Like that, but bigger, and data storage is much more… no, data storage is the same).
HDDs have the advantage of high-storage capacity. The disadvantage of HDDs is that they have moving parts within them. These moving parts are necessary to read and write data on different areas of the magnetic rings. There are two kinds of HDDs: Those that have worn out, and those that will wear out. That is, moving parts makes it prone to breaking.
SSDs have the advantage of no moving parts within them. This makes them much faster than HDDs because there are no moving parts trying to locate data. The disadvantage of SSDs is that the technology to manufacture them is limited, we are barely getting past 256GB. This makes them much more expensive.
As SSDs are RAM based technologies, another disadvantage is that if the drive fails to read data, then you have lost your data permanently. If a HDD fails to read data, the data can still be recovered by an external business (they have equipment to read the magnetic rings).
There are other pros and cons to each, but I won't go through them all.
What size SSD do you recommend?
I recommend a 1.2GB SSD — but they don't exist yet, so I guess you are up for an expensive 256GB one.