Rio or Buenos Aires
 

Where should we go? If you had a choice, which location would you choose and why?

Depends...
Buenos Aires - lots of history, best steaks in the whole world, tango, elegant/beautiful city
Rio - beutiful vistas from Corcovado, beach, tangas
Me? Buenos Aires.

For cultura, Buenos Aires. For beach Rio.

Preceding response is a good shorthand answer. Rio is much more crowded and skethier but has a good vibe if you are careful about your surroundings.
Also Brazil requires a $100 visa each (plus time to process) for us citizens.

Rio - beaches, beautiful combination of mountains, ocean, & bay, great people, wonderful food, lively atmosphere.