What do you mean by "arch"?
If you're referring to an incline press as opposed to a flat-bench press, the area of the pectorals that is being targeted varies from one exercise to the other. If you're referring to a dumbell fly as opposed to a dumbbell press, the stress being administered to the pectorals is different in both cases. The fly takes the triceps and shoulders out of the picture and forces you to put all your stock in the mighty pecs, whereas the press is a coumpound movement requiring total upper body commitment.
My opinion: Hit it with everything, bomb those pecs to oblivion.
I hope this helps.
Train hard and train smart.
Peace
