Last I checked the pectoral muscle is 1 muscle. There are no upper/lower middle pec exercise. If you activate the muscle, you activate the muscle. What determines how much of the muscle you recruit is the intensity of the contraction. What determines the intensity of the contraction is the weight. If you're not getting results, lift heavier weight. If you can't lift heavier weight, start lifting ballistically (ie slam that car trunk shut sorta movement). This sort of neural innervation temporarily recruits more muscle than a slow and steady neural innervation does. Which of the three bench presses (incline, flat, or decline) can you lift the most weight safely? I say flat bench because you can lift the most weight safely without dropping it on your ribs or face and do not need to rely on a bar, which would less activate stabilizers, which are obviously important to you if you care about the mythical upper and lower pecs. A spotter isn't as necessary for a horizontal press either as it is for a decline. I don't know anyone who can lift more weight on incline than decline or flat, so it's not even a good pec exercise but more of a shoulder exercise, in which case you may as well do overhead press or military press. If you still don't like the shape of your pecs, blame parents for poor aesthetics in terms of muscle origin and insertion shape.
Decline won't get you any different results than flat bench and is more dangerous.