Well it really depends on what you want to gain. If you want a big bench, well then you heav to bench sometimes. But, if you are looking to build the pecs like you say, I am going to assume you're talking about hypertrophy. You want growth, and I believe that dumbells are the way to go, but for flyes, not presses.
Another effective movement is the cheat fly. Take two dumbells as if you were going to do a dumbell press and as you lower the weights, let them drift outward slightly past your elbows. Keep them under control, as it easy to let the weights pull your arms out. Once you reach the bottom, reverse the motion, and repeat. You will have to find a weight somewhere less than what you press but more than what you use for flyes.
If you read "The Shoulder Horn" thread, then you should know that I find no fault in the bench press, however you really to do it correctly. Read some articles here on form; especially Dave Tate's stuff(can't go too wrong with a 600 lb bencher).
At the same time, don't make the newbie mistake of over-using the bench press. Most elite level bench pressers only do heavy, full-range bench presses every few weeks. The fill the extra time with other benching exrcises, like floor presses, board presses, rack lock-outs, and other variations.
Anyway, neither dumbells or barbells are really "better" than the other. If you have access to both, use them both. Learn to use both. There is a place in your training for both.
However, if for some reason you only can or want to use one, I guess I would have to say dumbells. They offer greatly versatility than a bar. You can do more with them in terms of exercises; and you more variations such as hand positions and range of motion.
But my real advice here is to use both.