I wouldn't worry about language in either place. Argentines are quick (too quick...) to point out that they are "Europeans" who just happen to live in South America, and are thus well-educated and understand a lot of English. They aren't really impressed if you speak Spanish anyway, although they so like it when foreigners call their Spanish "castellano" instead of "espaņol" (even though most of the Spanish immigrants came from Galicia and not Castilla). Brazilians, on the other hand, know that few people learn Portuguese and that most assume it is the same as Spanish. Most will be EXTREMELY impressed if you can just say hello and how are you to them. It's an easy way to start up a conversation with a Brazilian ;-)

Taking English classes is a status symbol in Brazil and pretty much anyone in the middle class has at one time or another taken a course, if for no other reason than because everyone else was doing it. The courses are not taught in schools or universities, so they are paying a lot of money for them...

I would recommend spending time in Rio as it is very beautiful and distinctively Latin American (while Bs As is deceptively European), and the people are a little warmer there than in Bs As. But definitely learn a few phrases in Portuguese.