Performance

That's an extremely complex question. It totally depends on what you're trying to achieve. If you're looking at large groups, and applying certain functions across all members of a group, then you might find that relation is less performant. You could probably get around that by using batch processing, but again, it depends on the function. On the other hand, relation is far more flexible than OG, so it may be more suitable for some uses. I think it's not really possible to answer this question as it stands.