User blog comment:YourPrivateNightmare/Discussing the new defensive mastery tree/@comment-24795478-20150626104616/@comment-5077844-20150626120006

Let me give you a calculation:

let's assume we got 200 armor and 100 Mr (bonus, not total) as our standard build (which isn't all that unlikely, given Frozen heart gives 100 armor, and you need at least 2 Mr items to get to 100)

if i now have 3 points in EA (which is what i do because i also want oppression) you'lll get:

+15 armor and+7.5 MR

if you put one of those points in AA you'll get:

+10 armor and +5 MR from EA

and + 8 Mr from AA

which kinda translates to: you either lose 5 points of armor, or ~5 points of MR

The thing is tho, usually you will take a lot more physical damage lategame, due to the ADC Dps, whereas AP mostly comes in the form of burst, which means on a tank the extra 5 MR might not matter all that much.

Let's put it that way. As long as you know that you'll need to buy MR at some point, EA seems the better choice to me.

If you're gonna focus mostly on Armor for a long time, then AA will probably be slightly more useful.