You want GVWR 1500 to 2000 pounds more than empty weight of the RV. Much less than 1500, you tend to overload. A whole lot more than 2000 pounds, ride will be rougher unless you can load close to GVWR; it is not like air suspensions, which adjust actual spring weights to a ride height.
There are not a lot of differences between to two chassis. E-350 once had a taller rear axle ratio, and option for 5.4 V8 rather than 6.8 V10. Now, with same engine, performance is equal. E-450 gets stronger rear brakes for extra 2000-3000 pounds of capacity, and higher recommended inflation pressures for same size tires.
