You need both.
Diet is the wrong word. Lifestyle change is the right idea.
You need to train your mind to look at all around you eating TBones, Onion Rings, chees, and French Fries and Dorritos and think "Total suckers". Curiously, these guys will say "Quality of Life would be lousy without these". But the real truth is the quality of life of those who are eating all this stuff is typically VERY low.
Re: Exercise. Most are too out of shape even think of real exercise. Start by walking 1 mile, and then build up to 5 miles walks. The weight will come right off, and then you can start working out for real (it will be much easier than the initial 2 mile walks!).
Most people are way too short term oriented. If I told guys that if they woke up, ate fruit, worked out for 1 hour hard, are chicken and salad for lunch, walked 3 miles, then had a reasonable dinnner and they could go out with a good looking girl and have a great time that evening they would do it WITHOUT THINKING. Heck, guys have hair transplant surgery, liposuction, etc. just to enhance their social chances somewhat.
But the rub is that 1 day of lifesytle change does not matter. You need 1 month to see results and people lack the willpower, discipline, ambition to make it happen.
Just say to yourself "Food is fuel" and get the Chicken whopper, no mayo, and not the ploppy square play, the 1200 calorie Taco Salad, or the terrible for you fried seafood dinner!