I always start with flat bench, followed by incline dumbbells, and then decline dumbbells to hit all regions of the chest. To feel the difference, just flex (even without weights, like now at your computer) to see which muscles engage. If you want to grow a muscle, target the exercise that activates it. For me, flat and incline emphasize my upper chest that needs work, so I focus more on those. If my lower chest required more attention, I'd adjust my routine accordingly. It's all about what you aim to achieve, but that simple flex test really indicates what each exercise impacts.