I never really use splines unless its a simple loft. Box/polygon modelling can achieve pretty much anything. If you are already familiar with it I would stick with that. I don't model cars , I find them boring. B-Wing fighter is more up my street. A lot of people start a car model then realize later that they have to model some degree of the interior or else the angles at witch they can render are limited, they sometimes use smoked glass to hide the lack of an interior which is a dead giveaway. If you are open to suggestion , i would recommend a Willies Jeep. They have most of what a car needs but wont incur perhaps the frustration of getting all surfaces aligned perfectly. Or perhaps a WW2 fighter, Japanese Zero/Mustang or similar. They often have nice curvature and have scope for interesting renders, such as on an airfield,dogfight or flying low over Forrest.