F350 - F450?

Looking at 2015 trucks not huge diference in price between 350 and 450. I have to step up to 4:30 gears with 450 where I can get away with 3:73 if I go 350. Will I notice much in the mileage area, this is mainly used for towing a toyhauler that can gross 18,000 if fully loaded usaully closer to 16,000 on most trips. I'm going from a 99 F350 2wd so i don't have much to compare with.

Thanks, Any help or thoughts appreciated!