Tube is measured to tighter tolerances than pipe. For example, a given wall thickness in tube will be much more uniform than a given wall thickness in pipe. My dad sells pipe for a living, and he told me that a lot of times you will find pipe that has really thin spots that can greatly lower the strength of the pipe. Tube is held to higher standards than pipe. Other than that, I don't know much about the differences, although I am sure there are plenty.
