If your Void Staff is penetrating 20 or more of the enemy MR, it's doing the same thing as an Abyssal, you're just not getting the 57 MR from Abyssal.
If your Void Staff would penetrate less than 20, then get an Abyssal.
In this case, a Void Staff is just as good of a buy offensively if your enemy has the magic 100 MR number; your marks, Sorc Shoes, and Guise penetrate 49, and then Void Staff penetrates about 21 MR, leaving the enemy with virtually 30 MR. If you have the mastery, then it takes them down to 28 MR.
If your enemy has less than 100 MR, then the Abyssal Scepter is probably a better buy.
Abyssal happens to be a decent item to rush if you have a Fiddlesticks or Ryze on your team, and can give you a nice advantage early on, because most enemies have 30 MR, which AS takes down to 10, and then a debuff from your team takes them down to 0.
Also, don't forget that Abyssal affects the entire enemy team.