Which hard disks for PostgreSQL?
That is a big fat "it depends on your application(s)". You say how large the total data is but not the commonly active data-set? All in one DB or several? If several is it the same structure in each (i.e. multi-tenant with one tenant per DB) or a mix of different application databases? What sort of read and write IO loads do the database(s) see and what level of concurrent requests. How much memory on the database server? If it is very read heavy and the data is small enough to fit in your RAM then disk choice is not going to make a lot of difference.

There are a few very general rules for databases:


SSD will trump traditional disks by a fair margin unless your data is large enough for that to be far too expensive. If your data is too large for SSDs to be affordable perhaps use partitioning (either manual by separating tables and/or databases between different drives and/or "real" partitioning to spread the data more granularly) to keep active data on the faster drives and archive data elsewhere.
As in the last point: separating data between different drives/arrays can help performance greatly in some cases even if the drives are all of the same performance levels, by reducing IO contention.
RAID for drive error protection and speed: RAID10 is generally recommended. R5 and R6 can have a performance issue for workloads with any significant write activity. Sometimes four drives are used as two independent RAID1 arrays instead of one RAID10 (two separate mirrored volumes instead of one striped set of two mirrors) to separate heavy IO onto specific disk sets - for some workloads this can be advantageous but it is more work to manage.
Have as much RAM as you can throw at it: if your data all fits in memory that'll be as fast as it can go.