Well, I've never participated in the Iron DM, but I have in the third ceramic DM and to a limited extent in the fourth (the fact that I had to withdraw midway is not something I'm proud of).
The Ceramic DM competition has the contestants create adventures and/or fiction based on a series of pictures selected for their hair-tearing, mind-numbing general oddity. Good fun. The Iron DM focuses on a series of ingredients given, from which the contestant writes up an adventure. Generally, there is a very limited amount of time in which to produce said works.
In some respects, I would agree with the observation you make; that the Iron DM is feels stricter then the Ceramic DM. However, I would say that the Ceramic DM is much tougher. Never having competed in the Iron DM, you can take what I say with a grain of salt. Yum, salt.
Anyway, both are much, much fun, and open to any and all contestants that sign up on time. Feel free to try your hand at the next round.