Not sure why exactly you are buying a trailer. But there are quite a few threads in Carfree living on this...particularly lately. Most seem to want something that you can use to haul groceries or other types of shopping.
My first thought would be to have a trailer that you could unhitch from the bike, push into the store, fill up, then proceed to the checkout. I was impressed with this wike for that
http://www.wicycle.com/gcs.htm
However, I suppose the additional unloading from the shopping cart to a rubbermaid type box on the Flatbed wouldn't be that much of a deal.
But if you are looking for votes, I would say go for the nomad. It looks like you could just put your bags right in... no need for an additional container.
Only problem is that you won't be able to use it to move a mini-fridge or something largish.