My comment had nothing to do with who Isco may or may not support, although SuperCity88 makes some great points on that.
I was referring to the fact that Barca and Madrid usually contribute about 80% of Spain's first team and bench, so if you have aspirations of playing for Spain, then moving to Madrid or Barca is a good idea.
Also, Madrid offered him the greater opportunity to win the Champions League and allowed him to remain in the country of his birth, where he knows the culture and speaks the language.