Best rear light is the largest one. Led tech has led to a lot of very tiny, but very bright, ultra lightweight flashers for road bikes. I see the flashing of course, but its still so small you have a very hard time judging where it is. Is it 50 feet away? Is it 50 yards away? It can be hard to say really.
You want them to see you, and be able to judge exactly where you are, about two square feet of reflector tape on your back will do the trick better. Makes you look large, something to not hit. Shows where your shoulders actually are if your arms are reflecting.
This helmet light looks good, simply because its not a tiny pinpoint. That it lights when you brake is good too, for riding in a place as crowded as he rides.