depends how you want to get about. There are several great malls around Miami, including the huge Sawgrass outlet mall, but you need a car. And apart from the art deco architecture along the beach the town itself isn't all that interesting. If you just like walking round the shops, then NYC is much the most walkable city in the country. Chicago and Boston aren't bad, and good places to visit, but NYC is bigger and better-shopped.
If you want to see a few sights as well (the majority of which have free entry) then Washington D.C. is the place to go. Not much of a shopper, so can't comment too much on them, but imagine it would have an outlet mall somewhere nearby.