South circular would be boring but doable. North circular would be idiotic in many places. There are some high flyovers, some long tunnels and cut through, many parts are 50 mph speed limit which means many people will do 60 mph, parts where you need to change lanes to stay on the correct road, slip ways coming on and off. I don't think bike are allowed on some parts of the North circular anyway.
South circular would be boring but doable. North circular would be idiotic in many places. There are some high flyovers, some long tunnels and cut through, many parts are 50 mph speed limit which means many people will do 60 mph, parts where you need to change lanes to stay on the correct road, slip ways coming on and off. I don't think bike are allowed on some parts of the North circular anyway.