I wouldn't go to Expanish, it has the most negative reviews (just an example, http://www.wordtravels.com/forum/discussion/6104/worst-spanish-school-in-buenos-aires-expanish-spanish-and-tefl/p1).
COINED and ECELA are pretty much the same, I know several students who were misplaced in groups that didn't match their Spanish levels, just because they didn't have a group of their level at the time (although they tell you they do before you pay, but right after you slide your credit card the smile wipes off their faces and they don't give a f**k about you).
I would go to a smaller school, these are usually the best and they are able to provide you with a more customized service, as it was mentioned above. I usually recommend BASP (www.baspanish.com) or Academia Buenos Aires (http://www.academiabuenosaires.com).