I studied Spanish at 2 different schools in BA, then again in Bolivia and then again in Colombia.
I put in a lot of research before choosing a school in BA, but neither of the ones I tried were right for me. It´s a very popular place to learn Spanish and I think there is a bit of a "formula" to the way many of the schools teach. I 1st went to a small one (which was more intense) but because it was small the group was very mixed, and as a complete beginner I was with 1 or 2 people who were fairly conversational. This simply did not work. So I tried a bigger school, where the group standard was a bit more consistent but the curriculum was poor. I didn´t learn a great deal but I enjoyed the school so I stayed, and I came out with some knowledge of grammar but very little else.
The I went to a school in Sucre, Bolivia, which was half the price I was paying in BA, for semi-private (not group), and the school seemed great. We got a little bored in Sucre and only stayed a couple of weeks but I´d recommend the school as good value for money.
However, it´s only now, studying in Colombia that I realise how much better classes can be. I´m going to probably the most reputable Universtiy in Colombia (EAFIT, Medellin) and the standard of the teaching is really good. There is still a curriculum but there´s less emphasis on grammar and more on conversation, which I think is really important. To be honest I think the grammar is quite easy to learn and you could almost do it in your own time using studyspanish.com or something. It´s the confidence to speak naturally and make mistakes which takes longer.
So, if money is tight there are cheaper place to do it (and Sucre is a lovely place, a little boring perhaps but that´s perfect if you want to study a lot). But if money is not a problem, BA is a great city to stay in for a while. Just be careful when selecting a school as I think it´s big business and the schools don´t need to be great to succeed.
I wasn´t sure whether to go to the University here as I could get private lessons for cheaper than my group ones at the Uni, but now I´m so glad I did. I finally feel like I´ve got a skilled teacher, which makes such a difference.
I haven´t been to Equador but I think Quito is probably a pretty good place to do it. I think they have a good accent and it´s cheap.
Good luck. By the way, I spent 3 months is BA and am heading back in about 2 months. It´s a very cool city and probably remains the one place in SA so far where I could live permantenly.