I can say something about commonly used Western Digital drives: WD has several flavors of its hard drives:
- Blue - mass-market - intended for mass market desktops/worstations (e.g. WD10EALX,WD10EAEX)
- Green - economic - intended for desktops/worstations with lower consumption (e.g. WD10EARX, WD10EZRX)
- Red - durability - dedicated for NAS devices (e.g. WD10EFRX)
- Black - heavy-duty - intended for servers (e.g. WD1002FAEX)
So if you intend to run your NAS 24/7 you should invest money in red drives (3 years warranty). They are silent, and have low consumption like green ones. If you don't need 24/7 then green ones (2 years warranty) fit well too. I bought 2x2TB green ones (WD20EARX) and they work fine. EARX are little bit slower (110MB/s) than EZRX (126MB/s) drives but work more silently and with lower consumption. However, I think that nobody should bother himself too much about speed (you will not reach this speed anyway in your NAS, no way), and differences between consumptions are really nothing, etc. Silent work, warranty, capacity (of course) and finally price - do matter.
If you want to compare and choose, maybe this helps: WD hard drives matrix comparison