Physically, there is little difference between the two trucks. The 350 SRW and 250 share the same frame, brakes and power train. What differentiates them is the standard suspension and and wheel choices, a 2" taller axle block on the rear of the 350 and higher GVWR rating due tires and springs. GCWR and tow ratings are same on both models.
Buying new, there should no reason to get the F250 unless your state charges extra for that higher GVWR. You can outfit the F250 with the same options as the F350 SRW, but the sticker will show a lower GVWR due to marketing. So, if you are concerned about price and registration fees, get the properly configured F250. If you are concerned about the GVWR sticker, get the F350 SRW.
If you are really looking at the next step up, then check out the F350 DRW. It has better braking and larger axle gears than the SRW's.
About 20 percent of the fifth wheel weight will be on the pin. This is why it is important to have sufficient GVWR to haul your GCWR. If your FW is 12,000 lbs loaded, it will carry 2400 lbs in the bed of the truck. You will also need to add the weight of passengers and other items you carry in the truck to see if the GVWR is sufficient for what you want tow. In general I suggest using maximum weight ratings for your calculations to get the worst case scenarios.
