What is the difference between mold in air duct and mold that grows in crawl space?
There are thousands of species of mold known to Science, with different species being found in different parts of the world.  There isn't any one "crawlspace" mold or "air duct" mold, it's just whatever spores of whatever species happened to take a foothold there.  Which one is more dangerous will depend on the particular species of mold and the sensitivity of the people that are exposed to it.  

In general, though, you're better off not having them growing in your home because even if you don't have a really dangerous one growing now, you obviously have the right conditions for mold so a dangerous one could start growing too.

For more information, have a look at this Wikipedia article on Mold health issues