questions in mind:
does 8gb single stick consumes less electricity over a pair of 4gb sticks? -
theoretically yes but it's negligible, you won't notice it.
which is more efficient? -
to some extent, less parts means less trouble and more efficient. but running 2 modules enable dual channel mode and that outweighs the disadvantage.
what are the benefits of a single 8gb ram? -
less parts, less clutter, less points of failure. disadvantage is if it fails, you'll have no other RAM to use.
what are the benefits of a 2x4gb ram sticks? -
dual channel mode. if one fail, you still have one spare... 
does it really matter? -
yes! dual channel mode proves to be beneficial in most cases than single channel.