A friend of mine uses a cargo trike, it is a homemade recumbent, and he uses electric assist to haul a considerable amount of weight.

http://drumbent.com/trike.html

Pros with something like this are you are quite visible, you can set up turn signals and brake lights etc. Very stable of course. The major con with it is its width - you cannot filter through congested traffic. So you get a bit more respect riding with traffic, but on the other hand, you have to ride with traffic. Depends on where you live whether that's a major slowdown issue or not.