Actually, I think my conclusion applies to anywhere you might want to study Spanish, not just Buenos Aires.
Don't go to a school. Get a private teacher. It very well may be cheaper and you'll learn Spanish better and faster than in a class with other students with whom you will probably be a mismatch with regards to level, language skills, motivation, etc.
If you're going to study Spanish in Buenos Aires, e-mail me and I'll give you the name of an outstanding teacher. If he's not available, he can set you up with someone who is.