I'll add my two cents and say it depends on what you mean by "offroad". If you're talking dirt roads and light trails, either will work great. The lack of articulation in the IFS is not a drawback until you get into serious rock crawling, and once you're at that point the size of either truck will work against you.
I think you're a little unclear on the purpose of those manual hubs though - they don't "lock" the front diff. They serve to disconnect the wheels from the half-axles so that you're not spinning the front driveshaft while driving around in 2WD, which gains a little efficiency and prevents a small amount of drivetrain wear. They're a stronger and more reliable setup than most modern electronic or vacuum actuated axle disconnects, but they don't "lock" the front diff, it's still "open". And while we're on the topic, I wouldn't ever spool a front diff or turning the truck would become a nightmare. I wouldn't even put an automatic locker in front. I'd pony up for an ARB or leave it open.
If I had to choose between just an F150 and a Superduty to wheel I'd take the F150 with it's slightly worse articulation every time, because if it came to it I could throw a solid axle underneath the F150 easier than I could make the Superduty smaller and lighter. Neither would be ideal for anything more than light trails - if you want to wheel hard and you're stuck on the idea of a Ford, look into a Ranger or Bronco, or even an early Explorer. I used to wheel a stock Explorer, took that little minivan lots of places it had no business being.