User blog comment:Obyvvvatel/Magic resistance isn't effective./@comment-4834593-20130516184044/@comment-4834593-20130516194054

^Depends on what you're looking at.

The flat penetration is boots and HG for Magic Peneration, both common items, and Brutalizer only for Armor Penetration, which is a slight bit less common (like, this one is build on about half the AD champions, while at least one of boots and HG will be bought on APs). However, percentage peneration (Void Staff vs Last Whisper) is, in my experience, bought for armor a lot more often than magic resistance. But that could be because there are no good magic resist items. I actually can't remember the last time I bought a Void Staff...