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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.
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.