The difference? 250 horsepower.
All kidding aside, model designations are not always a cut and dry as us modelers would like. On every diesel locomotive, the specification is continuously updated during the production period for any model designation. As a result the final specification for the F3 was virtually an F7 just as the final specification for the F7 was virtually an F9. When you get to model number changes there are more similarities than differences during that transition period.
As always, if prototype fidelity is of interest it pays to research the specific model you are interested in.
