Both work, but only one is a sustainable strategy.
Dieting down is like using an octane booster in your stock honda civic: it gives you a slight edge, but at the risk of burning up your car.
Eating to fuel your training is like rebuilding the engine: it takes more time and energy, but has way more potential to positively change your driving experience.
Spend most of your time building the engine.
Save the octane for the races that matter most, then use it sparingly.