Mission based with bonusses
What I like to do for xp attribution is to give it module based, with bonusses for great ideas or great roleplaying. It's a bit of a subjective manner of giving xp's, I usualy go with the feeling of the session (I give xp after each session), how much has been accomplished and how originally it was done. Like this scene's worth 200 xp, this scene's worth nothing, this scene's worth x for this player, but a bit less for another player. The differences depends more on players implication than on characters actions. If a mage character achieves a goal using a spell in an unconventionnal manner, he'll get more xp than the fighter who used his greater cleave feat to kill a few opponents...