As a Fuzion owner, maybe I can help you justify the price of the 355. Just looking at first glance, the Fuzion and Momentum trailers look very much the same. Heck everybody uses the same components to build these things. I'm still not sure I can justify all of the additional cost of the Momentum, but there are a lot of differences such as the use of 8,000 lb axles and >4,600 lb rated 17.5 tires and wheels. Grand Design seems to really insulate their trailers well. I have a hard time believing some of the R-value ratings especially since the walls don't seem to be thick enough to achieve their advertised ratings.
Overall there seems to be more attention to detail with Grand Design. I see a lot of little differences that indicate higher quality than my Fuzion. I also like that they have managed to squeeze more out of the basement storage than Keystone does. Higher end televisions and sound system are also nice benefits with the Momentum.
Nobody had heard of Grand Design over a year ago when I was a buyer and I'm starting to wish that wasn't the case. I don't know if I would have paid the premium to "upgrade" to the 355, but it surely would have been a consideration.