It is hard to define, for those not versed in SceneGraph lore.
OpenSG has some more clever multithreading features that OpenSceneGraph lacks. However, OpenSceneGraph seems to make up for it with a much larger and more vibrant community, and more extensive derivative and add-on tools.
I chose OpenSceneGraph myself nearly ten years ago and have not regretted the decision.
You might also consider more high-level game tools like OGRE, which might be more in line with what you want than either OpenSG or OSG.