As Tom listed, there is several differnces. What is important is what what you application is. Is the environment wet (THWN) or damp (THHN) and ambient temperture rating needed. A big one would be coduit fill. Given that THWN is rated at 75 degrees, wire derating becomes a bigger issue sooner then THHN. I typically use only THHN and XHHW. These two cover pretty much most typical applications so stocking is less of an issue at the warehouse and service rig. Much if not all the THHN I have used is THWN rated as well so either way, I am covered. I never specifically needed THWN so I have not really look at THWN in detail.
