The Marriott is a splendid hotel and the area is centrally located.....you get to sample the real Buenos Aires by staying there.

Puerto Madero is fine if you want to walk out of the hotel at night for a bite to eat or a beer.....it's a lively area popular with young professionals. Having said that Puerto Madero could be in any city in the world which has redeveloped their docklands so you do lose that feeling of being in Buenos Aires to some extent.

Truth be told it doesn't really matter as you'll have to use taxis to some extent no matter which one you stay at although the Marriott less so.