The mental side of Comrades is being able to deal with the pain, but in Ironman you have to concentrate for a lot longer. You have to train yourself to concentrate. I did a lot of mental exercises before Ironman. Ironman is tougher mentally because you know you're going to be out there for a long time, but Comrades is much more painful. When I've done Comrades I can't walk for a week, but after Ironman, I wasn't too stiff the next day.
Comrades is just about doing one action and using one set of muscles.
At least in Ironman you swim, which just uses your upper body, then you can relax a bit on the bike. But I must admit that the second half of the marathon at Ironman was probably as bad as Comrades. The thing with Ironman is the time it takes to train, because of the cycling. It's just one discipline and your whole body takes a pounding. This year was my first Ironman where I've cycled 180km.