Many paleontologists believe that Spinosaurus shown in JP III was completely fake and exaggerated.
And T-Rex is much more heavily built then any Spinosaurus species. T-Rex also has "bone crushing" teeth and T-Rex's single bite in real life could rip-off over 500 pounds of flesh from any victim.
Spinosaurus was mainly adapted at Fish eating. It's long jaw was good at catching large bony fishes or small creatures living in waters.