From what little I've read, the DR has had a somewhat less corrupt government over it's life; and Haiti has higher population density, going all the way back to having a larger slave population when it was a colony. In one of Jared Diamond's books, he makes the case that the DR has had less environmental damage from logging because one of the DR's past leaders was actually in the lumber business and prevented everyone except his buddies from logging public land. I assume the biggest issue is their history of corruption, resulting in more poverty and more overpopulation.