Nalia has 17 Int, compared to Aerie's 16. This means that Nalia should be able to learn more spells.
But I'd go with Aerie, her cleric ability is a big plus, and there are potions that can temporarily boost the Int stat that she can use to learn all the spells she needs.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
