That depends on a few things, SSDs need to be powered on every now and then to keep data, but as you are planning a 5 month backup cycle it would not be a problem, HDs don't suffer from this.
Second is price, HDs are cheaper in terms of storage space per dollar, at the moment at least, and would last a lot longer if it was left unpowered for a long period of time.
Third, you can get HDs of up to 10 gb, SSDs so far as i can see right now, are at 2tb for A$2000+
and fourth, as stated above, recovery is much easier on a HDD than an SSD
Speed can also be a factor, HDs are slower than SSDs