When the display goes into screensaver mode, the CCFL backlights remain on, so NO energy is saved -- regardless of what screensaver image you select. A solid black 'blank' screen consumes as much energy as anything else; the lamps behind the screen are still on, but the LCD panel has "pulled down the shades."
When the display goes into sleep mode, the backlight is turned off -- thus saving power -- and the screensaver settings have no effect. Sleep mode and screensaver mode are mutually exclusive.
Screensaver is a vestige of CRT days. With LCDs, it has no inherent advantage over sleep (other than the pretty pictures); however, for privacy, the OS-X UI allows you to set password-protection on the screensaver -- but (AFAIK) it doesn't offer password-protection for sleep.
Looby
