Since you're going for NAS storage setup, I would recommend the Red drive because WD Green is designed as a stand-alone storage drive, and also has save power and wear by spinning down when they can, which makes it a good choice for things like backup drives which are only in use periodically (e.g - once an hour).
WD Red on the other hand have RAID specific built-in features (like TLER – helps the drive recover faster in case of read/write error and the drive doesn’t drop off the RAID) that the Green drive doesn’t have. Their power consumption is really good, but unlike the Green which is designed to save power between uses, the Reds are designed to just spin constantly for continual use/availability. They're quiet, responsive, and have good speeds, and extended (3 year) warranty as well.
Hope this helps.
Cheers! :)