Overflows. Internal or External

[Madreefer]

I started to ask this in Kiens journal but decided I did'nt want to clutter his thread up.
What are people thoughts on external vs internal overflows? My tank is goin to have an internal coast to coast with herby drains, but it's going to be taking away 6" of valuable real estate from my fish and corals. But I dont want to have the tank out any farther than it already will be from the wall. Once setup I wont have access to the back of the tank (the outside)