I believe that it was the best option for Simon to cut the rope in order to make sure that at least one of them would survive to complete the task they desperately wanted to.
When Joe had made it back to the camp Joe had told Simon that what he did was the right thing to do and that he would’ve done the same thing if he were in his shoes. Joe still fiercely protects Simon's decision to cut the rope.
Lastly I think the Simon made the right choice because he needed to do what he needed to do. As horrible as that sound realistically there was pretty much no chance that Joe was going to survive. I also don't’ think that I can understand the rationale that one would have when they are in that situation because I have never had to make a choice like that ever in my