The e450 has more towing power and gets 8 mpg, compared to 9 for the e350. The e450 is heavier, uses stiffer springs and is 3 inches longer.
e350 is quieter and gets better mileage.
E-450 is not made in 138 inch
E350 has a 4.10 gear and the E450 has a 4.56.
Some owners have reported getting 10 mpg on flat terrain with steady speed.
They both have 6.8L V-10 has 305 HP at 4250 rpm and 420 foot-pounds of torque at 3,250 rpm
For a 25 foot motorhome the e-350 is fine, unless you are towing a car.
Photos of each: