We love our cargo bike (s)... sigh :)
We tried a trailer, I found them difficult to maneuver, through catwalks, and cut-thrus. They are also harder to pull up hills, and my bike wasn't geared low enough for the extra weight, we also struggled to carry gear with us fitting it in the trailer. Next I tried a rear bike seat on my hybrid, but when I added gear to the back rack as well, it was hard to maneuver, tippy front to back I would struggle at lower speeds and maneuvering particularly when walking the bike with only my 14 month old... the bike seat was not going to work when she was 4. At that point we purchased a long tail cargo bike. (Bonus the bike seat transferred to it) The lower back wheel, the gearing everything makes it a joy to ride even with lots of cargo. It is now my husbands main bike and we both use it interchangeably. We easily take 2 of our 3 kids on it in any combination (1, 3, 6), and can fit all three in a pinch. It carries a stroller, diaper bag, and picnic (including the kids) and tows my 6 year old's bike when she gets tired, or the roads are too busy for her skill set. I love this bike, we have about 300km on it since March alone.
As for the bike seat, I now often put it on my upright cruiser, it is much more stable then it was on my hybrid. I carry the baby, DH carries the 3 & 6 year olds and all the gear... we are about the same speed in this setup, I think he actually likes the extra exercise?
Yes I wish we had a battery assist, it is exhausting climbing out of the river valley with 100lbs of kids and cargo... but I can do it in the granny gear. If I'm tired - I can always kick the 6 year old off to walk, and since she knows this is the case she is often my biggest cheerleader chanting 'go mommy'... it's a huge improvement over 'why are we going so slow' from a few years ago!
As for our second cargo bike, we picked up a used bakfiet last year, it took all three kids, the baby in a carseat and the older ones on a bench in the box, with lots of room for cargo and a rear rack for bags to boot. They are long, and there is a learning curve to ride one - you need curb cuts for instance getting it up a square curve is difficult with any cargo. I am struggling to have the one year old in this bike this year as there are no straps to keep him sitting, so it's not very useful for this summer, but I foresee us using it next summer and for 3-5 years after that before selling it - I expect it will not depreciate very much, but may take a while to sell. I can see us keeping the longtail cargo bike forever.
So yes cargo bikes are awesome and worth the expense if you use them - we have definitely paid off most of our bike purchases just with miles traveled - if you can replace a car they very quickly pay for themselves. FWIW my kids love bike shopping, that is getting groceries by bike as opposed to buying bikes!