Mage Corps all the way. You are not getting 7s in three skills - you are getting 7's in one skill and two skill groups. And you still get the firearms group (and first aid, navigation, and unarmed combat), since they are from basic training (all branches get those skills). Special Forces gives you a bunch of rating: 1 skills that are far less useful - will you really use armorer, or free-fall, or pilot watercraft that much? If you have a compelling background-related reason for picking Special Forces, that's fine, but otherwise, Mage Corps is a far superior choice.