sancerre
529
Not weight
Believe it or not, the advantage is not weight, and certainly not cost.
However, the precision and capability to stay in adjustment are much better. For example, the front derailleur automatically adjusts based upon the chain location on the rear derailleur. This prevents the chain from rubbing against the metal housing of the front derailleur. On my (mechanical) derailleur, I have to do this manually, but there are only two possible positions per chain ring.
Oh wait, it is really cool, does that count as an advantage?
--Sancerre