I've got a double Chariot Cougar. It's definitely the best way to carry the load, even with 2 – fabulous piece of kit, they are secure and cosy, they can play with books, toys etc and if they drop them they stay at their feet or in their lap. Have been on all day rides with this, with stops and lunch etc. And when we have had two kids in it they do entertain each other nicely. It also makes the best pushchair I've ever used, on and off road 🙂
However on some gates and steps etc it can be a pain. I'd never use it on a main road, only on small side roads, and these can be a bit narrow. If I had a rear seat I'd be much more comfortable with the narrow streets, but less comfortable re handling and security.
Swings and roundabouts. If you've got wide pavements/bike trails and the roads are wide and easy going then trailer for absolute sure.