Ya hablas un poco de espanol?
Many foreigners prefer the South, although the northern parts are what I lovemost.
However, it depends on what you want and what you're bringing with you. If you already know Spanish and want to improve your skills there, I'd not go to Barcelona or Madrid, these cities are full of foreigners, you'd have trouble meeting a Spaniard.
Do you know how to find jobs there, do you have numbers etc? If I were you, and it's about the experience, I'd travel a lot.
Nice cities to see: Barcelona, Salamanca, Santiago de Compostela, Granada and San Sebastian (Donostia in Basque). If you're going to Santiago, you could also head down south to Portugal. Porto and Lisbon are well worth a visit. And the beaches of the Algarve are the best.