Seriously, I would never go back to one of the tourist schools personally, but your best bet I would think is to go along for a class or two and try a couple out. Things can change very quick and the teacher quality even in the schools can vary.
Even the school I went to let you sit in on a class for half the day (2 hours) to see if you fitted with the level etc.
Another thing I would recommend is NEVER UNDER ANY CIRCUMSTANCES pay for more than a week up front. A lot of the schools have people coming and going every day (including both teachers and students), your experience may change from day to day let alone week to week. Ignore the discounts, in the long run it may not be worth it.
The best thing about the spanish schools is the social side, but if you have the cash and other options to make friends/have a social life, you really cant go past a private teacher, one on one. Particularly once you have a little grammar under your belt.
To put it in to perspective price wise, say a school charges USD 200 a week and the classes are 17.5 hours a week (4 hours a day, 5 days a week less .5 hr break), it works out at about USD 12 an hour.
I am not sure exchange rate they use at the schools (assume its not official?), but my teacher (university qualified in lenguas, has a psychology degree and years of experience) charges 100 pesos an hour which at the official rate is about USD 16 an hour and at the current blue USD 12 an hour. And, I know there are very good teachers that teach at the schools that charge privates outside @ 80 pesos an hour (and I am sure there are even ones out there you could find for 60-70 pesos).