No, stick with Grail. You're losing like a total of 7 AP by grabbing it over Morellonomicon after his passive so it really won't make much of a difference. And the 400 extra gold is well worth the 25 magic resist and extra mana regen. So unless the enemy team has a lot of healing power, just grab Grail.
"I fear not the man who has practiced 10,000 kicks once, but I fear the man who has practiced one kick 10,000 times." -Bruce Lee LoL: TyphlosionShiny
By level 12 his mres per level makes up the difference, after that he's stronger than before. Do you know what viciously means? Or nerf?
Considering Athene's is the first item you build you should have it way before level 12 and by the time you are level 12 the MR isn't really as much of a big deal as it is early on in the laning phase. And I haven't even mentioned the mana regen aspect of it yet.