I think you're right - the main advantage of the trailer over cargo-bikes is that one of us can leave it at nursery and the other one can hitch it up on collection. I think the trailer is safe enough but then we've had M in it (well, in the Babyshell http://weber-products.de/zubeh/baby_main.htm ) since about 6 weeks with no helmet, so I'm clearly evil and bad and wrong :-), but I think the cargo-bike has got to be a bit safer overall.