Hello Ken,
I owned an 01 and an 03 ST 4x4 fully loaded. I now have an 05 F150 Supercrew 4x4 fully loaded.
Mileage 01 ST avg 17 mpg, highway 20 mpg, city 13 mpg
Mileage 03 ST avg 18 mpg, highway 21 mpg, city 14 mpg
Mileage 05 F150 avg 14 mpg, higway 18 mpg, city 13 mpg.
I personally went to the F150 because I needed more room in my box to haul around soil samples. I was just running out of room in the ST.
Pros of F150 - room, so much more room everywhere, 4x4 ability is better then that of the ST (muddy fields/trails/roads), the interior on a loaded one is really nice, a step up from the ST I feel. Insurance dropped a couple of hundred bucks.
Cons of F150, mileage is less, I am putting 110 cnd (97 US) into the fuel tank at a time and it doesn't take me much further than the ST did for less money.
PARKING, this thing is a monster, finding parking spots in the city can be a real problem. Underground parking garages in many places cause me antenna to scrape along the ceiling, plus turning in them is an issue as well. (the back-up sensors on the F150 are a must have)
That is all that I can think of from the top of my head. Like I said, the only reason I went to this bigger unit was because I needed the room. If I didn't need my truck for work I would be driving a car (a TDI jetta or something). If I hadn't needed the extra room I would likely have stuck with the ST. It is more of a happy medium for someone that doesn't need all the room that a full size has.
Edit: I just wanted to add that I have to drive me F150 very carefully compared to the ST to even get that mileage. Slow starts, keeping the RPM under 2000, not speeding. I got after ST a lot more and still had the mileage figures that I posted above. Any sort of headwind and you get city mileage figures on the highway. I drive about 75% highway 25% city/offroad. I could always get the ST around that 20 mark on the highway, but with the F150 all conditions must be right to get the 18 mpg. The best mileage I ever got out of the F150 was 19.5 MPG. That was one trip in the 35,000 miles I have put on this vehicle.