Those both look great. The child trailer I have at the moment doesn't have a collapsible seat, it sort of forms part of the structure of the trailer so I hadn't considered that option before. A double kid trailer seems like it would be a great load carrier size, I'll just have to make sure it's of a type where that can happen. How do you find the cargo on the floor? Does it sag down too much or is the canvas tight enough that this hasn't been a concern? I guess an option there would be to reinforce the base with some plywood cut to size, increasing the weight though.
The purpose built one also looks fantastic, it's something I need to consider as an investment for a new one as it's likely that I'll be getting a lot of use out of in the next few years. It appears to have pretty low sides, do you find that bungee cords are enough to keep the load stable and in place? I've got a lot of access to MUPs in my area, however they are all getting quite old and bumpy with tree roots growing under the path having significantly deformed the path surface in many areas with a resulting very bouncy ride.