Your question has a false premise. You appear to have been confused (deceived?) by WD's "IntelliPower" marketing gimmick.
In fact Caviar Green models spin at 5400 RPM, not 7200 RPM. Therefore, all other things being equal, the Black drive should have a much faster data transfer rate than the Green. The difference should be in proportion to their spin speeds, ie 7200:5400, or 4:3. In practice, the Green drives tend to get higher density platters, so this mitigates the disparity.
Unless a lot of companies have made mistakes I am seeing a lot of the new SATA 6Gb/s 64MB Caviar both Black and Green with spin speeds of 7200rpm. I am also baffled by seeing a 2TB black drive at £110 in price and the exact same Green drive from WD with the exact same spin speed priced a lot higher at one online retailer here in the UK. Wonder if that is a mistake for both? E.g. the price and the spin speed data for these two drives being both 7200rpm?
If correct it does still appear that the Black drives have an advantage because I notice latency on the latest ones down to around 4.2ms. That's pretty good IMHO. If you buy a few big drives and short stroke them in a raid set up I dare say even at 7200rpm that with these new 6Gb/s Caviar Black drives you're going to definitely blast anything out the water in terms of what Solid State Drives can do. You may not even need to raid it just format it to only use a small proportion of the space say 10 or 15% max and you will be spinning even faster.
I love WD drives they're great especially the Caviar Blacks. I've been a happy customer for a while now. I also think the Deskstar drives are pretty good abd very good value too if you need anything a tad cheaper, but they don't tend to make as many of those.
The green drives also go idle quite a bit (to conserve power). This causes problems for some RAID controllers. It also annoying when you need to access the drive and you have to wait a few seconds for it to spin back up. For the amount of power it saves, I'd rather buy the blacks.