I've never done a double IM, but I have done a regular one. Compared to other ultra events I've done that were just one thing, the IM was easier and more enjoyable. All ultra events are hard physically, but for me the single activity ultra stuff was way harder mentally, because I was just doing the same thing over and over for what seemed like forever. At least in an IM you only have to do one thing for X amount of hours, then you get to do something else. It breaks up the day into more manageable chunks, which for me was more enjoyable. I was exhausted at the end of the IM because I gave it my all, but I didn't cross the finish line hating swimming, biking, or running.

Double iron is less taxing on the body, especially so if we're comparing it to a 100 miler with significant elevation changes.