i think maggie because shes done some unexpected things in the series like knowing how to use a sniper with precise accuracy and look at her stewie will take her for granted.
Like average babies, Maggie is impressionable and easily influenced by what she sees around her. She once hit Homer on the head with a mallet, shot a suction dart at his picture, and brandished a broken bottle in The Simpsons Movie, as well as a pencil in imitation of Itchy and Scratchy. Despite her age, Maggie is a formidable marksman, as seen in Who Shot Mr. Burns? and Papa's Got a Brand New Badge, where she is able to shoot a whole mob of gangsters with a rifle.
Okay so I've had this discussing with stoners, alcoholics, professors, and just about every other walk of life that watches both. So here is in my opinion the best argument for the age o' question.
Stewie's best pro argument: Stewie's classic archetype of super mad-evil genius gives him the ability to alter the Space Time Continuum thus altering the fabric of Reality (I.E. the Q, the Force, etc...)
Stewie's best con argument: Stewie is also prevy to all the pittfalls of said archetype and then some. The short-list; megolomania, multiple sexuality issues, and not to mention nihilistic self-defeating complex. These pittfalls inevidibily lead to W.E.Cyotote syndrome (thus making Lois his Road Runner)
Maggie's best pro argument: Maggie blatantily has supernatural assasin abilities and the true killer instinct to match (I.E. Hanniabal Lector);however, she also possess the Superman gift (by which the collective will of the universe will not let the character perish for good).
Maggie's best con agrument: Maggie's suffers from developemental and social interaction issues. While it is clear that they do not affect her intellect, they do effect her judgement. These issues allow for chemical dependency, brainwashing, uncontrolable rage, and various other illicit activities. Her negative traits would allow her to be corrupted and loose focus (I.E. turned to the Dark Side).
So with this said, I feel it would come down to an Old West high noon stand off of the quick and the dead. Yes, Stewie will cheat, but Maggie will plan for it. This will then turn into a Standoff where Marge and Lois come in to take both babies off to naps. lol
I reckon Maggie's like the most powerful baby ever inside, she'll proberbly grow up to be like squirrel girls or something, Stewie will try and outsmart her and do the best he possibly can, but in the end, Maggie stares him to death will wile making that noise she makes whilst sucking on her dummy. Now THATS scary