I would highly recommend the Park Tower. It's one of the best run hotels in the system (they literally should send their top tier GMs for training there). If you're SPG plat, you will get a very very nice upgrade. As for hotel convenience, it's a good location, but not in the hip new Puerto Madero area across from Calatrava's cool bridge (where I recall the Hilton is).
Still, it's all a short cab ride away.
EDIT: Do check out the hotel's master thread:
http://www.flyertalk.com/forum/starw...-thread-4.html