Yes, there is a visible difference, though it is subtle. National and Continental stamps are hard papers, and American is soft porous paper, and will show fibers at the tips if the prefs. The real difference will be seen when held in front of bright light, Hard paper will appear translucent and the soft paper will appear mottled.
A bit harder to differentiate National and Continental stamps, but Continental added the "secret marks" to most of the series. http://www.stampsmarter.com/1847usa...etMarks.html