Powered controls - Electric/Hydraulic.
 

What are the pros and cons of the different ways of powering flight controls/flaps/slats?


For example the B747 uses (in primary mode) bleed air for the leading edge flaps and hydraulic power for the flight controls and trailing edge flaps. Whats the advantage of this setup instead of all hydraulic or all electric?


From what I have understood, on the most recently developed aircraft (A380/7E7) there is a tendency toward using electrical controls instead of hydraulic. I suppose it is to save weight, but if it is better why hasn't it been done earlier?