Both are essential on a mage. Get Rabadon's Deathcap if the enemies don't have much Magic resist and get Void Staff if they have built some Magic Resist items. Get both either way. ... However, most enemies will get MR later on in the game and there will be most of the time a spot for deathcap, so yeah, get them both :P.

When should I take void staff?

Void staff is gold efficient when the enemy has over 90 magic resist. If they do not (and will not) have over 90 magic resist you should probably not buy the item. As fiddlesticks, normally you want to ult fear the enemy carry and hopefully kill them.

How efficient is void staff?

Void Staff's base stats are 126. 14% gold efficient.