try barbell with a wider grip and really try to get a full stretch. I believe it is Warrior (I may be wrong, sorry if I am in this case) that talks about how you really cant exclude inner our outer pecs from one another, and that they will come hand in hand with one another. Just work heavy and make sure to always use good form and your outer chest will develope nicely.

A big thing with useing dumbbells is the angle of your arms when you are pressing it. You may not realize it but they could be angled rather then perpindicular to your chest and that could be holding back on your chest development.