Thats a tough call. The first thing I would ask myself, is either of these what we really want, or just ones that are a deal you came across. If not, it may be better to hold out for the next deal. It doesnt always seem like it at the time, but there is always another deal another day.
But, if they are too good of deals to pass up, then thats another story. What are you going to be using it for? Short weekend trips, travel the country, backwoods camping? How much camping do you do, or plan to do? 2-3 times a year, every weekend? How much time will you spend inside is another thing? If a lot, you want enough room for everything you need and to move around. If your like us and outside other than sleeping, then the shorter one is ok.
My suggestion would be, if you plan to camp a lot, make sure its something you will be happy with, all the way around. Towing, space, packing, sleeping, eating, living, pets, storage, etc....
If this is just a short mileage trip camper, few times a year, then the 18 footer may be plenty. We have a 19 foot now, and if it was only the wife and I, it would be plenty of camper for us. It is a tandem axle though, never hauled a single axle of travel trailer that long.