Next time you have it apart take it and push it down on a set of bathroom scales and see what weight it reada when it is fully compressed. Do the same with the other one you have and you can then compare and see which needs most force/pressure on it to compress it. That will give you an idea which is the stiffest.
Cant be sure but i seem to remember my stock was just under 40kg, 1000rpm rated was maybe just over or around 40kg, and my 1500rpm was around 45/47kg, but i may be wrong, it was ages ago i did it. Sure it was around those figures tho, or at least the differences between them was round that.