I would say Cartagena is probably safer.

I don't think there is much in it though.

In Cartagena you are unlikely to visit places other than the walled city area, Getsamani and Bocagrande neighbourhoods, which all have a strong police and security presence and are upmarket.

In Rio you may want to travel a bit more widely. Whilst upmarket areas like Barra, Lagoa and Ipanema are fairly OK, you may want to visit areas like Copocabana, Lapa and the centre. Although these areas are fine too and have a good police presence I have witnessed muggings and thefts in these areas.

Personally Colombia seems much safer to me than Brazil. But lots of posters here seem to post bad experiences. However I don't feel the need to be so on guard in Colombia than Brazil.

I have never been victim of crime in either country and have spent over three months travelling around each. I think it is best just to choose which appeals more and travel with a degree of caution so you dont attract attention and reduce risk.