I would argue that superpower involvement got the conflict started. The superpowers were the ones who allowed the State of Israel to be created in the first place. The US and the USSR then helped to perpetuate the conflict by supplying weapons to the two sides of the conflict. This allowed the conflict to start and become entrenched.
After that, non-state actors became more important. The main non-state actors were the various terrorist groups that formed in an effort to fight against Israel. Their actions, like the attack on the Munich Olympics and the various suicide bombings that have occurred in Israel, have helped to harden opinions on both sides.
If forced to choose which was more important, I would pick superpower involvement since it helped the whole conflict start and since non-state actors were not prominent for the first couple decades of the conflict.