tw cen futura paul renner twentieth century differences

Futura and Twentieth Century

I know I probably sound really stupid right now, but I can't identify between Tw Cen MT and Futura. If they were side by side, I could tell the difference (seems like Tw Cen is slightly heavier) however if I were just to look at a sign set in one or the other I wouldn't be able to tell what it was. What are some key differences?