I wondered the same thing, when I finally decided to register for a race. I debated doing the local ironman-distance race, or a proper Ironman.
In the end, I decided on the proper Ironman for a couple reasons. One: I wanted to say that I competed in an ironman. I figured if I'm going to spend so much money to compete in that kind of race, I want the support and attention a sanctioned Ironman race can give me, and I don't mind spending a little extra for that.