For me:
For falt bench: DB ALWAYS. DB is an open kinetic chain exercise. Bottom line is more stress is placed on the primary muscles (pecs) and other smaller muscles (delts, and smaller muscles of the rotator cuff) for stability. Barbell is a closed kinetic chain. Stabilizer muscles are used to a much smaller degree, and more of the stress from weight is placed on the joints (elboy, shoulder). Assuming you are not using too much weight, and proper form, DB is more effective and better for the joints.
As for Decline/Incline: Yes flat will hit both clavicular and sternal heads, but it does not FOCUS on the clavicular head, or lower fibers of the sternal head. The incline will isolate the clavicular head, and delcine will hit the lower, or distal fibers of the sternal head. (much the same way as dips). I would speculate that if Arnold was not doing declines, he was hitting the dips hard.
Just do all three, and get the most out of your chest workout.