Buenos Airies is absolutely class and the falls are definatley something else. I did know quite few people who got mugged when i was in rio, but nothing too violent! If you avoid the bad areas and don't look too much like a pasty western tourist you'll be alright. It's worth the risk to see the...
There's no denying Rio is dodgy, but don't let it deter you. Its an amazing place and you can be safe enough if you're sensible. Buenos Airies is the place to go though if you had to choose between the two.