Is sourdough more resistant to mold than other breads?
The short answer is yes, sourdough breads are generally more resistant to fungus due to the fermentation process of the sourdough starter. The reasons for this are only now becoming understood.  This study from the Journal of Applied and Environmental Microbiology says:


  Sourdough is different from traditional bread because it takes an
  extra fermentation step, which uses lactic acid bacteria to metabolize
  sugars and add that particular spunky flavor. The researchers found
  that during sourdough production, lactobacilli bacteria convert
  another acid found in bread flour--linoleic acid--into hydroxy fatty
  acids that resist fungus.


So there you have it, it's possibly better for you, tastes good (I think so), and lasts longer without going moldy.