If you do decide to pull the trigger on one... do your homework and know the differences between the 2. There are subtle differences:
F-450:
* Available only as a Dual Rear Wheel truck
* Only available with the full Super Crew cab, full 8' bed, and 4x4
* Better turning radius than any of the other Super Duty trucks because of the wider geometry front wheels. Helps a lot with backing & maneuverability... which is REALLY nice!
* Only rear end available is the 4.30... which will have no trouble towing anything anywhere, but will result in lower mpg. (I average 8 mpg pulling 16,000 lbs @ 65 mph)
F-350:
* Available as a Single Rear Wheel or Dual Rear Wheel truck
* Available in either Super Cab or Super Crew Cab configuration
* Available with either 6½' bed, or the full length 8' bed
* Available with either 4x2 or 4x4
* Available with gas or diesel engine
* Available with 3.55 rear end (Single rear wheel) or 3.73 rear end (Dual Rear Wheel)
I wanted a truck that that would pull just about anything I may decide to upgrade to in the future... which is one reason I went with the F-450. While the low 4.30 rear end doesn't get great fuel milage... that gear ratio is much easier on the engine when pulling high weight rigs up & down hilly areas. And it's also what gives the truck it's higher tow rating. Plus... with a truck that's over 20 feet long (263 inches front to rear)... I wanted the better turning radius the F-450's front end geometry provides.
In the dual rear wheel option, both the 350 & 450 are almost the same truck... the main difference being the 350 gets better fuel mileage, while the 450 handles better.
In my travels so far, I've met plenty of other RV'ers who are pulling with F-350 duallys who tell me they wished they had opted for the F-450 instead... again, simply because of the maneuverability. Of course, I've met others who are glad they have the better fuel milage... so it's a toss up.
Your particular requirements & preferences will be the key factor.
Here's a link for the towing specs on various Ford Super Duty trucks.... with tabs at the top for other specs. Good luck on whatever you decide!
2014 Ford Super Duty | View Towing Specifications | Ford.com