I have owned both.
As far as insurance, I added it to my RV policy, cheaper and better coverage.
Registration cost is dependent on the state.
Fuel economy with the bigger rear end cost about 10-15% depending on driving conditions.
9 1/2 shorter turning radius was a huge help maneuvering a 42’ 5th wheel
450 was not as high as the 350 so easier entry and exit
450 ride was much softer than the 350 (payload at the time was also slightly less than the 350 dually, but more than adequate). The 350 road great when hitched up...not so good empty. Huge advantage for the 450 for ride.