Think of it this way.
Hidan gets his head ripped off. He feels pain from it, but it's pleasurable and enjoyable to him. He gets a high off of it. He gets aroused off of it. Is he suffering? No, he isn't. He's feeling pain, yes, but he isn't suffering, he is enjoying it. It means that his pain isn't "painful" to him. It's actually enjoyable. He isn't suffering.
If Itachi got his head ripped off, he would be feeling an unimaginable amount of pain. Completely unbearable.
If they both got their heads ripped off, Itachi would be the one in the scenario that has the highest pain tolerance because he is feeling a concentration of pain that is unbearable.