I'd go with the bigger one if they are around the same price range.
They are not around the same price range, one is about $35 and one is about $60.
You do not want to use a WD Green as the primary boot drive. Storage, yes.
Yes, the 240gb SSD will be my primary boot drive, and either one I get will be storage. I will also use the SSD for a couple of my favorite applications.