That is a very good question. The 399TH was certainly at the top of the list. It is a beautiful camper and the fold-down side patio is an awesome idea. However, there are some key issues with the 399TH that took it out of consideration for us.
One of the main challenges with the 399TH is the orientation of the master suite. The bed in the 399TH tucks into the nose of the camper. We wanted a RV King bed. When jumping up to the King in a 399TH, GDRV has to put one side of the bed next to the wall. You can somewhat visualize this by looking at the 399TH floor plan pictured below. The box area that says King bed optional is where the edge of the bed sits. The location against the walk makes the bed a pain to make and the person who sleeps on that side has to crawl in and out. The bed in the 397TH is oriented across the bedroom in the side slide. This is an excellent design. A RV King bed has room on both sides to walk around.
The second and arguably the more critical issue is that the refrigerator cannot be accessed in a 399TH when the slides are in. When we are traveling we want to be able to grab a cold drink, make a sandwich, etc. It is a hassle and in some cases not an option to open the slides when traveling. The right side of the refrigerator in the 397TH is fully accessible when the slides are in.
Another key factor for us was the washer and dryer location. In the 397TH the laundry is in a closet in the master bedroom. This is an excellent location. The washer and dryer prep is in the middle of the garage for the 399TH. It takes up precious cargo space that we need to park our motorcycles. The 399TH is designed for a combination unit that has a washer and dryer as one device. Combined units have a terrible reputation. A stackable washer and dryer is another factor that made the 397TH the best choice for us.
The final item that looks awesome on the surface, but we were not sure it really is. I'm talking about the side deck. On paper it looks like a great idea, but in practice the deck is small and overhangs the camping site. We often visit campgrounds. When the side deck is down it takes up room on the campsite making it a pain to walk around. It has hard edges as well that can snag you easily if you are close to the deck. Getting on and off of the deck requires a big heavy stair that is not easy to store. The deck door doesn't have windows either. The big windows on the 399TH is on the service side of the camper, not the door side where most others are located. It didn't make sense to us to have the big windows on the coach overlooking our neighbor's outdoor space.
Here's the floor plan for the 397TH. When looking close at the two, the 397TH was the best choice for us.