LDT is a program that is on life support. It hasn't been changed or added to since at least the 2004 version, and Autodesk is threatening to pull the plug completely sometime in the next few years.
C3D is still having growing pains, and is at-best still a "beta solution", but I can't really recommend learning LDT at this point, if you have any say in the decision. They both have a learning curve, but different ones, so learning LDT won't really help you learn C3D. And C3D is significantly more-powerful than LDT. Once you get used to it and familiar with all the current problems, you will be able to do things faster and better than in Land Desktop.