I always buy used, always have and if I ever wanted to change our rig I would buy used again. Making smart decisions on what to buy and investing the time to maintain your truck and trailer to a high degree has always seemed prudent to me. Whether it be be boats, trailer, trucks or motorcycles my overall cost of ownership has always been low. If cost is not an issue for you then go new and enjoy the large range of choices. Buying used limits your choices and may require learning to like a less than perfect floor plan. Also, I do my own work and maintenance. If I had to pay to have stuff done then maybe the differential between used an new would change.
There has been a progression in fifth wheels and trucks over recent years to get tall. Sometimes the used trailer/new truck combo will not work due to height differences. Like kstanton we bought a used Domani. We love the trailer but I don't see it fitting a new truck very well which would limit the pool of future buyers. Good luck and enjoy the search.