When I was writing HARP one of the things that did not want to do was to make the giving away of XP based upon combat, as there all sorts of different campaign styles. I wanted something that would work for all the different styles equally well.
My solution - Goal Based XP.
There are 4 basic types of goals (Major Party Goal, Minor Party Goal, Major Personal Goal, Minor Personal Goal), and almost anything can be given as a goal (rescue the princess, defeat the dragon, steal the lost treasure of xanippi, negotiate a peace treaty, etc....)
Goals are also given a difficulty rating, and the number of XP gained is based on that difficulty rating. However, it is important to note that the Difficulty Rating of Goals is not set and is based on how the characters solve/resolve/complete the goal. This means that the Difficulty is set after the goal has been accomplished.
Thus, a group whose goal of rescuing a princess from the dragon could go about it various ways. One is the direct approach and while this may end up be Very Hard or Extremely Hard, another group who uses a sneaky approach may end up giving them more minor goals in the process. Having more minor goals may make each minor goal not quite as difficult, but the more complex the plan, the greater the overall difficulty (because of the greater chance of something going wrong).
Basically, the GM determines, after the fact, how difficult it was for the goal to be accomplished for the PC(s).
A side benefit of this is that the player's personal goals can also be used to drive the story/campaign as the GM inserts leads/clues which can eventually allow the player to complete the personal goal as well.
Personally, I just really like the idea of goal based. Please note that a "mission" could actually be made up of several different goals, so it has a bit in common with mission-based rewards.