A mage recieves a +2 stat boost to their primary casting stat.  Most Mage's spellcasting  ability stems from research and knowledge (INT), however some rely instead on their sense of intuition, casting spells almost by instinct (WIS).  Other Magi rely on pure force of personality (CHA).  In the end, it is more about flavor.  Spellcraft will always use the primary casting stat.

 

The following primary casting stats are suggested based on the Magus main sphere: