WD drives tend to burn out and start clicking and losing sectors early. I know because I have had 5 different models from all the way back to 2004. History repeats itself for them I guess lol.
I've had no problems with Seagate drives though. Last year they had a couple of bad batches (isolated incident) but nothing since then. Just be sure to read the reviews on newegg of the specific drive in the NAS before you buy.
If you aren't going to be using it constantly then the WD might work for you.
Edit: I have always been a heavy gamer and also test different Linux distributions often, so all those read/writes are what killed em for me. Had better luck with Seagate but YMMV.