Algodoo

I believe the difference in the spring constants between the two assemblies is due to the fact that the spring on the right assembly is attached at the midpoint of the blue arm, but the rope on the left assembly makes contact at a tangent point on the green pulley that is the radius distance of the green pulley away from the center point of its blue arm. Maybe that difference is enough to require a different spring constant.(?)

Sometimes it's easier to calculate using energy instead of forces. The 5 N load is lifted 8 meters, so that's 40 Joules. The spring energy required to do that is 0.5*k*x^2. Solving for k, k = 2*PE/x^2. For the right case, PE=40, x=4, and k=5. For the left case, PE=40, x=8, and k=1.25. I'm thinking that the 0.05 N/m increase in the left k is to overcome autoBend=true in the rope hinges plus friction between the rope and pulley surface.

I concur. And I came to that conclusion experimentally by adjusting the friction of the various components that interact with each other and observing the results.