Quintessense of Greater Warding = 4.74 MR
Quintessense of Resilience = 4.74 armor
EDIT: I use Malphite as a tank (who doesnt?) but just had to add that...
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It's decent, AR + MR will benefit from Malphite's passive. Although more AR will strengthen his thrid skill. In the end it's really up to you and your playstyle.
I personally though, don't invest on defensive runes, my theory is that you're not always the target esp. when you're playing as a tank. But I have to admit, I play Malphite as an attacker sometimes, it's fun. Gotta love Armor Pen + Brutal strikes.