Willem,
I was hoping this would draw more comments than it has. I am curious about the same thing. All that I know is Van's official statement that they are not aware of any performance differences between the 72 and the 74" props. So I guess no one has tested one against the other. My guess is that there's probably not an appreciable difference, but it would be good to have confirmation.
Van appears to recommend that if you have a nose-wheel aircraft go with the 74" and if tail wheel, go with the 72", and this seems to imply that there's a slight advantage to the larger diameter. Don't know.