Flies are arguably better at isolating the chest because they take triceps completely out equation and yes, better rom for the chest (unless you have a converging chest press, which my gym does and it's GREAT). Your front delt is active in both presses and flies since it helps with horizontal adduction of the upper arm. There's no way to keep it out of the movement. However your chest will fatigue before your front delts will in flies. Your pecs want to bring your upper arm in, up, or down and all movements that move your upper arm like that will work your pecs. (once you get too far up the front delt really takes over but you really only have to worry about that with a very high incline press)