If you are putting your OS on the drive, I recommend a WD caviar black edition for better performance. If the two above ar your only choices, then I'd go with the blue. I know the green is supposed to vary it's speed, but tests show it runs at 5400 rpm most of the time anyhow.
I would recommend the Samsung Spinpoint F3 1TB over the WD Black for a cost savings... IMO If you don't need huge storage, the Samsung Spinpoint F4 320GB drive is a very nice OS due to it's speed. As stated above, if these drives are avaiable to you.