Hi there lElprince,
It really depends on what is the purpose of the drive. Regarding the two WD models that you have mentioned, I would say that WD Black is designed to be a primary drive with good performance. It is a premium product that comes with 5 years of limited warranty. In case you need a secondary storage drive, WD Green may be the better option as it has power efficient characteristics and runs quiet.
Hope this clarified the things a bit,
D_Know_WD