If you bought an 250 now, when you go to buy the 5th wheel, you could find that a 3/4 ton is too small, unless you're planning on buying a smaller FW.
Money aside, towing a TT of that size and weight would be so much easier with an F250. It's not just the weight of the TT, you could end with a lot of weight in the TV, esp. on a longer trip. I would not be surprised to find that you are over on payload cap. now. The larger truck would feel a lot more comfortable to tow with. The brakes alone make a big difference and more so in hilly/mountainous terrain. I don't think mpg would necessarily change much and may depend on factors like diff. ratio. Unless you really, really want/need 4WD I would not get it - it reduces your payload cap. a lot and you're higher off the ground (not great IMHO).
We went from an F150 to F250 last year and are towing a 29' TT weighing nearly 7K lbs. The difference is night and day and so much better, I'd never go back to an F150. Until you've tried it, you can't appreciate the difference. We actually get better gas mileage with the 3/4 ton despite having a V10. It is a lot more stable and there's a lot less sway. No regrets...
What to do? No idea.... It depends.
