Dangi is correct. The green is more focused on saving energy, and simply storing data that doesn't need to be accessed quickly. The black is more speed oriented.
In terms of the cache, the green's 64MB will speed it up a bit, but probably not enough to catch up to the black, even with the 32MB. They do make blacks with 64MB though, I guess they just don't sell them on that site.
There's also the RE series drives, which are enterprise drives. They're a little more expensive but they're much more reliable, and just as fast, if not faster, than blacks. Currently, I could only find them in SATA 2 (3Gb/s) but most standard HDDs don't quite max out SATA 2 yet, maybe in RAID they would.
Another thing to consider is that blacks and RE4s come with a 5 year warranty, greens only come with a 2 year warranty. That's a big plus considering how iiffy HDDs can be, and right around 4 years is when they start to become unreliable.
Also, a lot of this depends on what you need the drive for. If you're just storing pictures, movies and documents, a green would be the best choice. But if you'll be loading games and OS files off the drive, a black or RE4 would be a better option.
EDIT - A lot of this repeats what Chief said, I guess we were typing at the same time. And check out the WD site for more info: http://www.wdc.com/en/products/internal/desktop/