* Vehicle type and cost: A 2-door sports car will almost certainly be more expensive to insure than a 4-door family sedan, even if they are of similar age and condition. The sports car will have a higher value, likely higher repair costs, and is statistically more likely to be involved in accidents (due to performance capabilities and driver demographics).
* Safety features: The presence of advanced safety features like airbags, anti-lock brakes, and electronic stability control will significantly influence the insurance cost, regardless of the number of doors.
* Driver profile: The driver's age, driving history (accidents, tickets), location, and credit score are major factors in determining insurance premiums. A young, inexperienced driver in a 2-door car will pay far more than an older, experienced driver in a 4-door car, even if both cars are otherwise identical.
* Repair costs: The cost to repair damage to a vehicle varies significantly depending on the make, model, and availability of parts. A 2-door car might have more expensive repairs if the parts are rarer or more difficult to source, though this isn't always the case.
In short, the number of doors is a minor factor compared to others that dictate insurance premiums. It's impossible to say definitively which will be cheaper without knowing the specific details of both vehicles and the driver.
