Completion EXP could work just fine, but would probably not be implemented since it would add a means of leveling in addition to leves and "force" min-maxers to quest in addition to grinding if it's significant. The way this works in most other MMO's is that a quest has a level, and experience decays as you outlevel the quest (for instance, a rank 20 quest gives 100% SP at 18-22, 50% at 23-26, 0 at 27+).

I still think the best solution is to just have the material rewards of the quest scale with player rank. Rank 50 quests would just award Gil or armor in proportion to their difficulty.