When (or "why") is it better purchasing Void Staff instead of a second Rabadon's Deathcap? Without the passive, Rabadon's Deathcap gives 120 AP! I think It's more than the Magic Resistance of most kinds of champions (with the exception of tanks).
Personally I think getting a second rabadons is a bad choice. There are other items that give a lot of AP (close to 120) plus you get lots of other stuff as well (for example Deathfire Grasp which gives you 100 AP and 15% cooldown reduction and a great active which is like one extra spell). The reason rabadons is so great is not the 120 ap. It's the passive.
So in my opinion you don't have to get into the whole magic resistance vs magic penetration calculation.
If your dilemma is Second Rabadons vs Void Staff - Just go Void Staff.
But if you want a truly damage based comparison overlooking every other aspect of the game lets do some math using this website: http://idunnolol.com/lol/calculator.html
If you have
You will get
with void staff you got 35% magic pen so you get 65 magic resist and 70 ap so you have
and you get
So its even when the opponent has 100 magic resist. If he has more magic resist void staff beats rabadons if he has less rabadons beats void staff
also if your spell has more Ap Ratio than 1.8 Rabadons wins if your spell has less Ap Ratio Void Staff Wins.
You can do the math for any spell you want. But if you dont need void staff then I still insist you get something else instead of Rabadons.