If you are going to be towing a load, definitely get the big truck. They're a lot more stable, and fuel usage will be about the same.
I have a Ford Ranger, which is essentially the same as the sport trac, with the same 4.0 V6, and the mileage with no load is nothing to brag about. Nice little truck, easy to park, very reliable, but not much when it comes to towing. Glad I kept my old F150.