I used BigWigs through all of BC and found it to do everything I needed. Just before Wrath dropped we re-tooled our raiding system and had this very same discussion amongst the officers and raid leads. We basically came to the conclusion that Both were effective.
I tried out DMB shortly after our first few Naxx 25 clears. I liked the feature some have mentioned with boss health bars for Thaddius Twins and 4 Horsmen, but in a reality Main Tank targets do the same thing, so that canceled out for me. Probably the biggest thing I liked was the auto whisper when someone tries to whisper you while in a boss encounter. But really that's the only thing I found to be "great". It had an interesting feature that told you how much you improved time wise on a previous kill of the same boss, it was cool, but hardly a deal maker.
So I went back to BigWigs. I dont know if its just what Im used to but I like it so much better. Feels more customizable. I love the global bars I can create as a raid lead for breaks. Kinda sucks it only broadcasts to other BW users tho.
In the end I would say it comes down to personal taste.
There is something so appealing about backhanding someone across the face with a shield.