[wingnut60]

Welcome to the 5thwheelforums...

Since you don't have either of the units at this time, it is good to be asking questions. First, is the 12000lbs the advertised DRY weight or the GVWR (Gross Vehicle Weight Rating)? Big difference, as the DRY weight can be as much as 3-4000lbs less than the gross weight (gross weight = how much the unit is designed to weigh at max loading), and most of the mfgs build to minimum standards, not maximum. If you will be fulltimers, then you will have it loaded with all necessities for living, not just a week away from home--all this 'stuff' will weigh a lot more than you realize. Do you plan on a lot of camping without hookups--if so, you will need a generator to keep your batteries charged.
Second, if you are set on a Ford, get the F350 instead of the F250--almost identical units with more towing/load capacity, with little more in cost.
Third, if you are looking at 35' length, you will have to be thinking LITE/LITEWEIGHT and that will probably lower the durability for fulltiming. Fulltiming puts a lot more wear and tear on a fifth wheel, expecially if you move a lot rather than stay in one place. If looking at the shorter lengths, I don't think there are many mfgs that have one at that size that is appropriate for fulltiming. One option vital for fulltiming is a washer and dryer, either separate units or a combo, and that takes up room that would normally be a storage closet--so the 35-36-37-38' lengths will more likely be what you need.
Having said all the above, have seen many couples with smaller fivers/TTs, etc having a good time with less than all the above, so it really comes down to what you and your partner are willing to live with. My wife and I have been on several months-long trips and have realized that fulltiming would not be for us--hope that whatever you come up with for traveling is the correct choice first, rather than have to upgrade/upsize to meet your needs.
Driving habits, speeds, hours each day will come to you as you travel--only you can set your own comfort level.
Best of luck in your search.

Joe