I have a third Mk-17 that is installed in a CL Flying Clown. It currently has the small venturi installed, just because the plane is small and I did not need the extra power.
Perhaps the larger venturi would be good if you were using a small prop and needed higher power at higher RPM. Perhaps the small venturi would be needed if using a larger prop where the lower RPM would not produce less velocity in the venturi. These are only speculations, that's why I started with "perhaps."
I had no trouble bench running them with either venturi, but only the one is in a plane. Fortunately, so far none of mine are wet nosed.
I remember reading in a review that Ed Carlson included in his catalog, that the BB's are instrument grade. Not necessary, but a nice touch.

George