I'd say buy based on pricing/warranty and make sure that the drives are tested and approved by your NAS manufacturer.

I only use Seagate drives in my NAS and have been quite happy with them since 2009. I started with 4 1.5TB drives (yeah, THOSE 1.5TB) and then upgraded to 6 2TB drives. I've replaced 2 drives that started to grow errors as reported by SMART, the second one only a few weeks ago.

The only WD Green drive I have (in use) is a 2GB that I originally bought to test in the NAS but it just didn't behave, so I ended up using it in my Apple Time Capsule. It died and was replaced under warranty. The replacement seems to be doing fine at the moment, back in the Time Capsule. The original Time Capsule drive is also a WD - 500GB Caviar, which sits in an anti-static bag on a shelf as a sort of emergency backup. I can't really think of anything to do with drives under 2TB anymore. The 1.5's all live in my PVR server now.