I honestly wouldn't stay in Puerto Madero. It's a business district located nowhere near anything, and to get in/out on foot you have to walk across train tracks and cross very broad and busy roads, which is extremely unpleasant due to all the traffic and the fact that locals drive like maniacs.

For a first-timer I'd recommend staying either in Recoleta or on/by the north end of 9 de Julio, or else for a quieter, more neighborhood feel stay somewhere in Palermo.