Picture an omni source - it hits all the walls, near far what ever, on the first wave. Now picture the directional source, picture it very narow, like a beam of light. It hits only one wall on the first pass, another on the second, maybe two, its at least three or four reflection before the reverb field begins to build. The omni happens immediately.

This takes a lot of thought, but think it through and you'll see what I mean.

So I am saying that one type is definately better than the other.