Kettlebells work better than dumbbells for flyes. Because of the position of the resistance, the pecs are under load for a greater proportion of the range of motion.
With dumbbells, the muscle tension on the pecs is greatly reduced at about 60% of the range of motion. This can make the reps less effective, especially when you're using a method where you must maintain constant tension to create an occlusion effect.
A perfect warm-up virtually ensures a perfect workout, but a poor one will almost certainly ruin what COULD have been a great training experience. Ever wake up dreading the idea of going to the gym, but after getting there, you end up having a great workout? You can thank your warm-up for that.