from my experience, the trax editor is better for moving keyframes around in order to get timing right with different objects... at the same time, the graph editor can do the same, but instead of moving keyframes (although you can do that too) it helps alter the ease-in and ease-out of each motion. if you think a motion is going too fast or too slow you can alter the actual rate of movement in the same amount of time instead of moving the keyframes and making said action take less time or more time. originally when i would edit in the graph editor i would turn off all tangent weights and watch the animation, then turn them back on per keyframe to adjust the build-up or slow-down of a motion. it's like imagining a pitcher on the mound or a car at a starting line in the fast and the furious... they each start up with a slow motion and then as they reach the end of their movement they get faster and faster... that would be something to use the graph editor for. the trax editor would be best used for moving keyframes of one (or several) objects because they don't take enough time or take too long to complete their motion. although this is just how i used the different editors, i'm sure others in the forums would be able to give you greater insight to the positives and negatives for both the graph and trax editor.

anyone care to add to or correct my statement?