[Nerull]

In early game, I would probably go with Viconia, just because you won't have as many spell slots to fill with buffs as you will later on. Thus, Viconia wearing armor will make her stronger than Aerie as a tank in the beginning. However, once you get the right equipment on Aerie (I disagree about the Robe of the Good Archmagi; she is just plain awesome with the Robe of Vecna). She will surpass Viconia, especially with some levels under her (i.e. more spell slots for buffs). However, when going against opponents that like to dispel magic a lot, those extra buffs for Aerie won't last long.

Honestly, I would go with another character as your tank (Mazzy or even Jaheira, if you are going all female). That's not to say you could use Aerie or Viconia as tanks, but having to cast all of those buffs before tanking is rather high maintenance.