LED's can more effectively hit the certain spectrum plants like cannabis need to grow. Certain Red and Blue LED's hit the exact wavelengths that promote growth, while cutting out other unnecessary wavelengths to save power (and money). It takes a lot of LED's though to put out anywhere close enough to the amount of lumens a plant needs, so LED ballasts are usually only used as extra lights to supplement a main light source like CFL's or HID's.
leds are expensive but produce very little heat hids are cheap but require lots of extra equipment cfls are cheap but small and u will need lots of them