It's hard to pick, on one side GP4 has accurate tracks, nice AI, and a good race atmosphere. F1C has a better feel of freedom, better default graphics and 4 seasons to choose from.
GP4 feels like it drives on rails, is still quite unstable with modding, CPU-usage heavy, quite limited in many technical aspects (too many to list), and a few other things.
F1C feels like it's being driven on ice, has a weird GFX system that stretches cars, horrendous damage model, limited GFX capabilities and you can't save your race at different times of your race weekend.
In common, both need mods to survive.
To start, hold down launch control and once the lights are off, let go off the throttle and press it all the way again quickly.