Q: I want to lose weight. What will help me more - diet or exercise?

A: Both diet and exercise are important for successful weight loss. If I absolutely had to pick one of them, I would definitely say exercise is more important, both for health and for cosmetic reasons. Without exercise your body will not be compelled to keep muscle over fat when losing weight so in the end you would still have much of the fat you wanted to lose. Instead you would have lost a lot of muscle and some bone mass (both of which are desirable from both health and looks perspectives).

On the other hand, exercising alone is not guaranteed to get you any weight loss at all (in case you consistently overeat even after taking exercise recovery needs into account). But if you eat average (not too much, not too little, and well-balanced) and exercise you will likely reach or at least approach your goals -- it would just take much much longer than if you complemented your exercise routine with an appropriate diet.