I think the only place where barbells have the clear advantage is in the ability to adjust load weights in small increments, and the ability to load more weight. Heavy DB's are hard to find in most gyms. A 200 DB is practically unheard of unless you have it custom built (prepare to pay BIG cash). Even then, in order to truly effectively progress you would need to be able to move plates on and off yourself, and I think that IronMind is the only company that makes a DB that will handle that kind of weight. And if memory serves it costs a pretty penny. If you stick to Olympic plates as your standard, you will need a lot of tens or fives to make up the weight, as a 25lb Oly plate is too big to be used effectively.

So while I like DB's and think they serve their purpose, I also think that it is very difficult to utilize them to develop strength past a beginner stage. Not impossible, but hard and costly.