Buenos Aires is way more expensive but of course it is a world class city and it a very much more developed country than Brazil. Rio is tiny compared to BA and really a beach resort town. The vast majority of the population as you know from visiting there live in the shanty towns surrounding the city.

Having said that...you will find model potential in each. Probably easier in Rio since you can go to the beach and what you see is what you get (as in everyone is pretty much naked). Not so in BA.

You will also find more dark skinned guys in Rio. The men in BA tend to be on the pale side.

Lee