Is their bigger bragging rights in doing an MDot organised Ironman race over a non MDot race? And if you complete a non MDot race, can you still truly call yourself an Ironman?
There are two controlling organisations: The ITU and IronMan. IronMan is a trademark of IronMan... there are subtle differences in the rules (not as much as the difference between Rugby league and rugby union though!).
IronMan just does longer distances (Half/70.3 Ironman, and Full IronMan). ITU does sprints, Olympic etc. middle distance = 70.3, long distance = IM.
Whatever you do, brag away!
Really, the quality of the race can be measured by the quality of your peers.