BB flex is not material dependent as much as design dependent it would seem to me. You design for stiffness desired by controlling the tube diameter, shape and thickness in that area for the characteristics of the material used. Even different steels have different characteristics which have to be properly accomodated in the design.

When I upgraded my crankset some years back, I noticed a major difference in stiffness to my surprise. Both old and new were aluminum. The old had solid crank arms, the new were hollow, bigger diameter plus a little lighter. The old crankset was a cheaper unit that came with the bike.

Al