The primary reason that D-T is considered to be more easily viable than others is that it has the best numbers under the Lawson criterion. This is also true under the Triple Product test. While Wikipedia gives a good summary I can't find a better reference that is online (The Wikipedia article gives references including Lawson's original paper but I can't find any of them online). The real advantage of He3 Deuterium fusion is that it is aneutronic, that is it doesn't produce any neutrons. This means that there's much less nasty radiation that will harm the containment vessel and other parts and that much less of the energy will be in difficult to capture forms. This is especially important for magnetic confinement since neutrons lack of charge makes them not confined by electromagnetic fields. This is a non-technical article that discusses a lot of the basic issues including the distinction between fusion types, although they don't go through the level of detail of actually using Lawson's equation.