Side panniers ? rear luggage rack.
Those of us that are fortunate to be in jurisdictions where filtering is legal, a top box is so much easier than panniers, because narrower is better. It’s also much better for pillions that have an unreasonable fear of coming off the back. You can also remove a top box and just strap thing down on the carrier plate. Those with side bag please correct me if I’m wrong, but you can’t fit a helmet in them, can you?
Having said all that, saddlebags do look better than a top box. No idea which is more efficient, those with them have said you lose about 10% range with the panniers, but my untested logic is that a top box is already within the disturbed air behind the rider and shouldn’t create as much drag.