The Green Drives have a unique firmware on them which monitors usage and optimizes their RPM speed. Most raid arrays have no way to deal with this kind of thing. For this reason I'd go with the WD Black drives.
The RPM speed of the Green drives is not adjusted during usage. The speed is fixed during the manufacturing process of the drive and can not be altered.
This possible difference in drive speed is actually perfectly fine for RAID controllers (read controller not array, the array doesn't care), especially software ones. Hardware RAID, however, does have a problem with these drives, as they do not support TLER. The same applies for the Black drives, though, although they can still be TLER enabled, a feature WD completely ditched for their Green line.
If the OP wants to use a hardware RAID controller, enterprise grade drives really are the only sound option. For software RAID, especially under OS X, any WD drive is perfectly fine. Black drives do give better performance, no doubt, but they are also considerably louder. I replaced all the Black drives inside my Pro with Greens after only a couple months as I couldn't stand the noise any more. With larger capacity drives (I replaced 1TB Blacks with 2TB Greens), the performance actually remained identical.
I can confirm 100% that these drives do indeed spin down when in a Raid.
They do also spin down when not in a RAID.
However, this can be avoided by de-selecting the "put the hard disc(s) to sleep when possible" option in the energy saver preference pane. Naturally, this works only if OS X handles the array.
I've got two arrays of Green drives in my Pro (both RAID0) and neither does spin down. They do after about half an hour of me not using the computer, but while working, they keep spinning. I've actually never seen a beachball or experienced any kind of slow downs compared to the Black array. It's exactly the same.