Difficult choice, but I personally think that Simba suffered more than Nala.
In Simba's favor, he obviously really suffered when he saw his own father die, and had so much problems with it by forgetting the past. Also, Scar forced him to leave his own home, and that has to hurt, because he also had to leave his loved family. In another hand, he suffered too when the hyenas tried to kill him before he walked into the desert; He fell on a lot of spikes and left damaged I guess. Then he walked alone in the desert, without any food or water in hours, before Timon and Pumbaa saved him.
Instead, in Nala's favor I would say she suffered when she believed that her best friend Simba was dead. I guess she also suffered by living in Scar's Reign, with destroyed PrideLands where there wasn't any food or water during her teenager ages.