We haven't run any benchmarks, but with the atom processor having an SSD is a reasonal call.
This is my personal experience:
Reliability: SSD > HDD > SDCARD
Speed: SSD > HDD > SDCARD
Affordability: SDCARD > HDD > SSD
If you need to store large files which you don't need to access too often then HDD might be a cost affective solution.
SDCARD/USB-pendrive is only good for portability, it is not designed for high throuhput. So use them for non-critical data only.