TLDR: Ancient's Grace is not a competitive choice to Willpower as a 3-piece jewelry set. It should be, and can, by increasing its 2-piece bonus to +5% from +4%.
At first glance, players can see the +4% max magicka buff from Ancient's Grace and think, "Awesome! I bet that's a lot." People are already used to putting Inner Light on their bar if they have extra space, and that's +5%, so why wouldn't you want +4% too?
Ok, well first of all, Inner Light isn't just +5%. If you have your Mage's Guild passives unlocked, it's actually +7%, almost double what Ancient's Grace provides.
Secondly, 4% of 40k is only 1600. That's not a huge amount. Now you may say, "but my max magicka is already 50k so I'll get 2k from +4%." Not so fast! Because the percentage bonus is additive to other bonuses, that means it's only 4% of your BASE max magicka before any other % bonuses are considered. So that 10% high elf racial passive, that 6% from wearing 5-1-1, or things like Inner Light or Bound Aegis, none of it counts towards the amount that gets the +4%.
So lets take a best case scenario for max magicka:
5 Necropotence, 2 mismatched magicka monster set pieces, Master's staff, Mage mundus with 7 divines, blue health/magicka food.
We'll also consider these bonuses in light of the HotR update...
Base max magicka with 64 attribute points = 15062
1096 x 6 (3 from Necro, 2 from Monsters, 1 from Ancient's Grace) = 6576
4000 (Necro with pet)
868 x 3 (big piece enchants) = 2604
351 x 4 (small piece enchants) = 1404
870 × 3 (gold jewelry enchants) = 2610
1032 (Master's Staff)
100 CP in the Blue Tree: This applies a multiplier to the above stats, roughly between 16-16.5%. The % increases, the more magicka you have. The Mundus and food buffs are NOT augmented by this CP bonus. It is also not effected by other % modifiers.
3092 (Mage mundus with 7 divines)
4936 (blue food)
15062 + 6576 + 4000 + 2604 + 1404 + 2610 + 1032 = 33288
33288 x 1.165 = 38780
38780 + 3092 + 4936 = 46808
Doing pretty much everything you can to stack max magicka, you end up with 46808 magicka that can be boosted by the +4%. So the max bonus you could ever reasonably get would be 1872.
Now let's add up our other % bonuses:
10% for high elf passive
6% for Undaunted
7% for Inner Light
8% for Bound Aegis
2% for Shooting Star
That's 33% added together
46808 x 1.33 = 62254 (without the +4%)
Adding the +4% gets us to +37%
46808 x 1.37 = 64126
64126 - 62254 = 1872 added, like we expected.
Now let's compare this to Willpower. We are looking at Gold Ancient's Grace vs. Purple Willpower (since Gold Willpower is not available)
840 × 3 = 2520 (from Arcane trait)
1400 magicka
186 spell damage
On a character with 2005 spell damage, the 3-piece bonus of Willpower actually adds 201 spell damage, while using Major Sorcery increases the bonus to 239. For argument's sake, let's round down our average spell damage from this bonus to 230. We can multiply this by 10.5 to see it relative to max magicka:
230 × 10.5 = 2415
So if we consider all of the benefit from Willpower in terms of max magicka:
2520 + 1400 + (2415) = 6335
The actual value of magicka gained would be 3920, but the damage increase would be equivalent to gaining 6335 magicka.
And this is what we got for the best case scenario of Ancient's Grace:
870 × 3 = 2610
1096 from the 2-piece bonus
+4% (reasonable maximum of 1872)
2610 + 1096 + 1872 = 5578
On a build that is not just stacking max magicka, but achieving more of a balance, a base magicka of around 35k is more likely. This would provide a bonus of 1400.
2610 + 1096 + 1400 = 5106
So, while the max magicka is indeed higher in Ancient's Grace, the actual damage potential is quite a bit lower, partly because spell damage bonuses can be amplified while % bonuses cannot.
SUGGESTION:
To make Ancient's Grace more competitive with Willpower, the % bonus should be increased to 5%.
Let's look at the Ancient's Grace numbers again with a 5% increase:
2610 + 1096 + 2340 = 6046 (with 46808 magicka build)
2610 + 1096 + 1750 = 5456 (with 35k magicka build)
These number are still below Willpower's 6335, but are much closer, and because max resources also help with sustain and shield size (which spell damage doesn't) it's reasonable that a set that only boosts max magicka should not overtake one that mixes max magicka and spell damage. A substantial bonus also incentivizes building into max magicka to gain more benefit from it, as opposed to using a set with raw stats. My argument is that the gulf between Willpower and Ancient's Grace in terms of wholestic effectiveness is too wide, and can be brought to a more balanced place by increasing Ancient's Grace to +5%.
@ZOS_GinaBruno