The technical difference is that on the wet sump, the gears run fully immersed in oil. On a dry sump, oil is injected at the gears contact point and then suctioned from below and into a holding tank. The real-world advantage is you lose the drag associated with turning the gears in a very viscous fluid. That equates to a decent amount of "found" horsepower.
As has been said before, when you get to a certain point, a dry 6 is the cheapest 50 to 75 horsepower you can ever find. That point is probably around 1k/HP.