I'm not so sure. Cuba was propped up by Russia just as North Korea has been propped up by China. Also, South America seems more susceptible to coups than many other parts of the world (e.g., Allende in Chile). Chavez is courting suitors (e.g., Iran) to pick up the slack when foreign investment dries up. However, Iran has it's own set of problems and I see that "friendship" probably less close than Chavez would like. Kicking his neighbor in the teeth just doesn't seem like a smart move.
Cuba has been on its own since the Soviet Union died, and that’s 17 years. Moreover, Fidel Castro, the cult of personality who has been running the show, is at death’s door but still was able to hand over power to his brother Raul without incident. His other pupil is of course Chavez. As for North Korea, China can only do so much without Kim Jong Il serving as a puppet ruler, and he isn’t.