Not an easy answer for sure. I love to do both but it really depends on you. Setting up with a 135 pound barbell and 2 65 pound dumbbells is not the same, and the dumbells get harder and harder to setup as the weight goes up. I really think my biceps issue is a result of setting up with 135s, so having help to at least get them set on your lap and not having to pull them off the floor like a deadlift, curl them up the last inch or so, find the bench behind you so that when you sit your in the right spot, then the layback and lifting the weight into place above you while not falling off one side or the other, smashing your chest as you fall back, and hyperextending the pec or putting your shoulder into an odd position for the first press, is really helpful. (yes you have to say that last part with one breath!) That said, my bench really started to climb with the addition of heavy for me (135 on a straight bar) skull crushers and heavy for me (270 pound - 135 each) dumbbell presses.
One thing that is really nice about the DBs is the ability to work each side independently. So we do what are called Brian's, because of a beast at the gym named Brian who started doing these. With a relative heavy weight start in the normal position with the weight at the bottom of the press, perform alternating single arm presses for 5 reps, then with both arms perform 5 reps and hold the weight at the top of the last rep and perform single alternating presses for 5 reps. 3 or 4 sets of these will really abuse the pecs!