The disclaimer: I do not actively use my truck to tow stuff every day of the week. I very rarely use my trucks to their fullest capability. The most I've pulled with any of my trucks probably weighed about 6000 lbs, vehicle and trailer combined.
That being said, F150 versus F250, the answer is, it depends. For my uses everyday, my '04 F150 is more than enough truck. If you're going to be doing some fairly heavy towing often, I'd always step up to the larger truck. Bigger brakes, stronger frame, more capable load carrying suspension. Short of something that says Kenworth or Frieghtliner on it, I don't think there's such a thing as overkill on a tow rig.