I think Pohnpei has it pretty straight. They certainly developed much of the infrastructure of the country, but they also got their massive profits by cutting corners, ignoring the safety and well-being of their workers, and paying the lowest wages possible for incredibly hard work.
The answer to this is "yes." They were both. It just depends on who you ask.
These men were captains of industry. The businesses they made helped to make America prosperous. But they were robber barons too. They got monstrously rich, partly by somewhat shady practices.
So these men did help America, but they did not always do so in nice ways. Therefore, they were both robber barons and captains of industry.
This is a challenging issue to address. On one hand, I think that the presence of the industrialist in the latter half of the 19th Century was vital to the growth of the nation. The movement from farms to factories and the onslaught of industrialization that took force in post- Civil War America could not have been fully accomplished without the industrialist. In this light, he can be seen as a "hero of industry." These individuals had the keen insight to understand and comprehend what America was and will it to what should be. At the same time, I think that most industrialists of the time period had to be seen in some light as "robber barons." This was a time period where there was little in way of government regulation and social perception sided with the industrialist in that there was distinct primacy placed on making money. The industrialists of the time period were not overtly concerned with wage regulation, ensuring that workers' rights were respected and helped to drive the reality that economic wealth and material prosperity was valued above all. In this light, I think that both visions have to be assessed when examining the industrialist in American History.