User blog comment:YourPrivateNightmare/Discussing the new defensive mastery tree/@comment-973379-20150626183531

AA give 4% of highest resist always. EA give 2.5 of it and 2.5 of low one(i.e. if you have 200 bonus armor and 40 bonus m.r. - say runes+mercs for mr and FH+randuin+runes for armor - 1 point if EA will give total of 6 def stats, while AA - 8).

Say they have AD top, Zed mid, Seju jungle and some random support+AD bot). And you are tank. Probably you will build heavy armor(+mercs), neglecting m.r... But this m.r. will still be very helpfull against Seju and, probably, support. And some magic damage from top(like Garen ult). And may be ADC too.

Also thiis will almost always give m.r. - and m.r. is harder to buy than armor.

In perfectly balanced build(150 bonus armor and m.r. EA is better, but with. say, 200 armor/100 m.r. 8>7.5)