You should definitely choose the Florida Mall. For teens and kids it is the only mall in town that has Abercrombie & Fitch, abercrombie (for tweens), Hollister, American Eagle and Aeropostale all in one mall. Ask for a mall discount book at either concierge desk although no discounts for the above stores.
The Mall at Millenia has the above stores too except no abercrombie or Aeropostale. However, it does have Orlando's only Urban Outfitters which is a must see for that age group. The Forever 21 is also four times larger than the one at Florida Mall.
A&F prices are much cheaper in the U.S. than the stores they have in Canada. We were in your Toronto store this past summer and a pair of wide leg jeans my wife had just purchased here in Orlando was C$20 higher in Toronto. I've not been to a Canadian Hollister so can't answer that but it's likely cheaper here.
There is a brand new Rue 21 at the Prime Outlets International, the rebuilt version of what was the old Belz Outlet World.
BobK/Orlando
