I agree with all the others on the floorplan thing. My wife and I looked at many units, sat in them, pored over brochures, then went to dealers and looked at them all over again. Sat in them, imagined ourselves living in them, etc. We were fortunate that the floorplan we loved the most early on is the one we ended up purchasing.
Also, I agree not to grab something based on either "our truck will pull it" or "it's smaller, so it will tow easier". Be honest, go back to paragraph one, sit in them, imagine yourself storing all your stuff in it, see if you can even walk around the bed (that was a big one for us). Get what fits you.
Also, I would agree that you want to get all the options you want up front (ordering a unit is ideal). We bought ours off the lot and spent some time and money adding things like a level-up system, etc. that we wanted. However, we had a specific reason for purchasing off the lot as opposed to ordering. But, if time is on your side, get it right the first time.