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If you intend to travel with six people, I strongly recommend you re-consider your feelings towards the 31-footers. On a rainy day, it will seem a lot smaller than you can imagine...
Oh, and it's my opinion that they are cheaper, too, since until they realize that the big trailers are just as easy to tow (some say easier) than the smaller ones, a lot of new buyers avoid them, thus reducing their cost.