While it may be livelier, it is "new restaurant" lively, i.e., you don't have the cornucopia of cafes, shops, etc. out your door. My place is a short walk from the other (non-Hilton) end of Puerto Madero, but in the oldest part of the city, not in the newest. I'd rather be in the oldest.

The Libertador is arguably the most centrally situated for getting to all parts, but it is a fairly-worn hotel, albeit with nice staff. It is on a very congested street where you don't have time to think when you leave the building. The other hotels near the Plaza will provide a more relaxed feel when coming/going. I've never stayed at the main Sheraton. I try and avoid the mega convention and cruiseshiphell hotels when possible.

Gaucho makes some good suggestions. That is, if you are important enough for the Alvear Palace But with Starwood in the mix, definitely consider the Park Tower.