Dual coils will generally give you more vapor. Regarding flavor, some prefer single, others prefer dual.
The difference between the old and the new style dual coils is that in the old ones, the wicks are visible and stick out. The new ones have small holes that you can see some wicking material through.
Single coils also have wicks that stick out, but only one end sticking out on each side, as opposed to the old style dual coils that have two wicks stacked on top of each other.