The decline excercises (decline bench, high-to-low cable flies) will hit your lower chest. As far as whether dbs are better than bar, I think it's important to incorporate both in any workout.
The chest stuff will build and tone your chest muscles, but it won't do shit for any fat that's already there. From your stats, you're obviously not overweight, so the chest for you would be a "problem area." That fat will be very tough to get rid of. I have the same problem. I have the center line and veins popping out of the top of my chest and shoulders, but the lower chest has a pocket of fat that fucking ruins it.
Anyway, try some different chest stuff. Go heavy for a while, see if that works. And, depending on how important it is to you, you might look into getting some surgery.