I've heard that once an AP gets his/her Void staff, they should sell their socc. shoes for merc because the void staff reduces the effectiveness of the socc. shoes thus its better to have merc.

Anyways, can someone confirm this for me?
Thanks in advance.

Well what iut REALLY does is reduce MR after applying flat penetration.

So if they have 100 MR

w/ Sorc: 80
w/ VS: 60
w/ Both: 48

The question then becomes... Is the 12 MPen worth the slot? Probably not in a long game.

Edit: If you have MPen mastery then you lose another MPen off sorc boots. And you already "lost " 2 effective MPen to start with.

Seeing numbers helps out a lot.
Thanks dude.
Edit: Just to ask another question, what choices do i have after I sell the socc. shoes?
Instead of mercs, should I get like a lich bane or FON to make up for the lack of move speed or just stick to the mercs?

Situational, depending on your CDR/map control/warding/buff control, Lucidity boots are often the best option for a caster that needs to stay mobile. FoN is rarely appropriate for an AP, the other common choices are Mercury Treads + Lich Bane. Occasionally QSS is mandated, because just one hard CC will lose you the teamfight.

I agree that you shouldnt have scorcerers shoes if you have void staff... similarily, if you plan to purchase void staff, you might want to replace your magic pen runes with ability power runes, since they will contribute to more damage throughout the game, and you can get the msot out of your void staff and runes.

It depends what champion you're playing, but the boots you might want to purchase are situational. You might want defensive (tabi or merc treads), or lucidity if you havent already maxxed cdr, or mobility if its one of those boring late-game-turtle games and you want to move around the map faster for farming etc.