Mike Finch asked a few survivors of both events. Around the world, the Ironman is regarded as the toughest one-day endurance challenge. This is what the experts had to say. But it's more for the guy who has skill and technique. You need to swim well and bike well to be strong on the run.
Comrades is for the guys who are tougher.
Even mentally Comrades is tougher. I know a lady who had only done one triathlon in her life and decided to do Ironman. She only found out two weeks before that she couldn't do the race on a mountain bike and had to borrow a bike to compete. In the end she completed the event easily. You couldn't do that in Comrades.