HDD = Slower, larger and are a KNOW quality (i.e they WILL fail, but we know from experience should last decades if they don't have a mechanical failure )
SDD = Fast, very very fast, but can fail suddenly, and have a finite life (SSDs WILL fail eventually, they have a certain number of read/write cycles, then they will cease to reliably store data)
Personally ill take the mechanical drive over SSD for data storage anyday, id NEVER store anything i want to keep on an SSD, they are still too new to be a proven storage method.. Mechanical HDDs are cheap and large enough to stick in a raid, and give space + redundancy, SSDs are "getting" cheap enough to Mirror and put your boot drive on.