From my considerable experience with fishing, any monofilament line will stretch. That's why on the sport boats that go for the big tunas, etc. and use 80 to 100 lb. test line, the crew just about insists that you replace your line every night. For me, I'd use twisted steel cable for my pull-pull. I'm currently building a Funaero SE5a and am ustin the cable on my rudder and elevator pull-pull. I've done this in the past on my planes and have not been disappointed yet.