I lived in Lagoa and Ipanema for a few years and would walk each day all over zona sul which is a big area. Personally, I never suffered an incident and felt "safe enough" and that I enjoyed a high quality of life there, but I did see more than my fair share of incidents including:
- a shoot-out with automatic weapons between police and kidnappers who hijacked a taxi in front of shopping Leblon as they sped away down the main street of Ipanema... some bystanders were shot and wounded
- being stuck in gridlock traffic in a tunnel to see hundreds of people come running, screaming in terror through the tunnel as criminals had entered the tunnel and were robbing motorists at gunpoint
- seeing a similar seen on the beach in Ipanema, as criminals were robbing beachgoers at gunpoint
- this kind of mass robbery tactic is common enough it actually has its own word and definitely one to learn if you plan on living in Brazil: arrastão.
- a dead body lying in the middle of the running path in front of the lagoon, most people including families with children casually walking around it
- a bank robbery in Ipanema where they literally blew up the bank with a bomb to gain access one night - it literally shook me awake
- Many people I know who live in areas like Barra and Recreio have directly suffered violent crimes including home invasions and kidnappings over the years - these are less likely to occur in the more densely populated south zone where the biggest issue is usually opportunistic street crime. Pretty much everyone I know there has their "stories" to tell regarding crime in that city.
The worst thing in retrospect is that it becomes normal after a very short time and very easy to shrug off what is happening to others around you.
Frankly speaking there is zero comparison between the two cities in terms of safety. Rio is one of the most violent big cities in the world, there is no sugarcoating it. It is a city where each year over 200 people are hit by stray bullets (not even targets of crime) over 1000 people a year are killed by the police and more than 2000 people fall victim to murder.
If none of this sounds like what you know from Buenos Aires, you have your comparison.
If you want to look at numbers, then that one city approximately accounts for more murders than
all of Argentina each year.