Direct lighting, as in those provided by clear glazing will provide plants with more intense light as long as the plants are in the direct path of the light. Anything beyond the direct path of the light will not benefit from the same light intensity.
If your light source is coming in from just one direction (ie: southern sun), then you would notice that there are pronounced shadows in areas where the direct light is blocked.
The benefit of direct light is that you can grow more light-demanding plants, but at a limited space in the growing area, since not all spaces in the greenhouse may receive the same direct light.

Diffused lighting (translucent glazing/shade cloth, etc) will scatter the light coming into the growing area so that it covers a wider field, although with less intensity. Since the incoming light breaks up into different directions, there will be less "complete dark spots" in the growing area.
The benefit here is that you can utilize more space in the greenhouse, but at the cost of not being able to provide the maximum amount of direct sunlight to light-demanding plants.

You could come up with a hybrid between the two glazing options, depending on what you want to grow in your greenhouse.
For example, have one glazing panel as clear so that you have a "window" on that side that offers direct light for some plants, while the rest of the panels are translucent for general purpose growing.

There are several "Greenhouse Gardening" books available and would be able to provide more detailed information regarding the choice of glazing material to achieve the light levels that you need.

I don't think scattering reduces the light significantly - it just looks that way.

Diffuse light is meant to be better for plants in a greenhouse.

I was all for clear glazing, as clear as I could get it because when I was in my greenhouse I wanted to see out!

So did my seedlings....

Diffuse light in a greenhouse is definitely better. The light entering is the same but when the sun is at a low angle plants don't lean south (or north) when the light is diffused.

(the things one learns by accident, my glazing was crystal clear, misting with water high in minerals turned my covering milky white inside... I cannot see outside, but the plants are growing up instead of sideways.)

Just my opinion.

My glazing started out clear as well, but the same thing occured, minerals from the water (we have well water high in minerals), and the algae that tends to build up because of the high humidity (I clean it twice a year) have made my glazing mure translucent on the walls, but not on the ceiling. The only real shade in my greenhouse comes from surrounding trees that cast shadows as the sun walks over the sky and hits everything at different angles. My plants do grow straight up, though :-)

Diffused light may also inprove photosynthetic efficiency.

Here is a link that might be useful: Diffused light study

Thanks for the responses everyone. The specifics of my problem is this: I will be growing very light-hungry trees with dense canopies, and they don't like shady spots...any part shaded will stop growing. I think I've found the material I need. It'll provide 85% light transmission with 100% light diffusion...the best of both worlds...I hope.