He's the one who gets to snap at Mac and snark at their therapist and bristle at the Secret Service guy, roll around in a wheelchair and an arm-cast, and react to the relentless hounding of his own angel and devil doppelgangers. John Hodgman gets all the good lines, all the great costumes, all the most absurd and hilarious moments.
He's kind and earnest, sincere and occasionally befuddled, always trying to help out his friend, but rarely does anything more than stuff his hands in his pockets and rattle off a few of his own best features and benefits. He is - and this is the true, ironic brilliance of the conceit - the straight man.
Justin Long exudes an effortlessly cool, understated, charismatic vibe - he is playing a Mac, after all - but he never really has all that much to do.
3) PC is the undisputed star of the commercials.