2 months in South or Central America???? SANTIAGO???

Hello,

I spent 2 months in Buenos Aires last year and studied Spanish.  I have 2 months this year as well and would like to return to a Spanish-speaking country in order to further gain fluency.  I am 27 year old female student and would be traveling alone.

Does anyone have advice of a different city to go to?  My priorities are:

1) safe
2) big city (esp since I will be there 8 weeks)
3) good public transportation
4) relatively low cost of living (i am a student)

Is Santiago Chile very expensive???
Is Mexico City dangerous? (People say it is; however, I do not know whether that is a skewed western notion of danger based on the media?)

Any other cities?

Thank you!!!

Roxana