i first went to Rio five years ago. when i got there i was errified by everythign i had read, but i quickly got over it. the people there are so nice and the place is so beautiful. all you need to do is try to blend in - just put on a bathings uit and sandals and head to the beach - wear nothing that will attract attention (flashy jewelry, watches, etc), carry just a little bit of money (leaving your wallet and credit cards in the hotel safe) and you're fine.
Buenos Aires is entirely safe. i lived there for a year and have been back several times, i have never felt in danger at all. common sense, the same kind that you would apply in any big city, is all you need. again, another amazing, beautiful city with the nicest people in the world!