The former is the US edition of the book, the latter is the UK edition – there is essentially no difference. Mitter looks chiefly at the Second-Sino Japanese War and from a relatively political angle, so if you want something that has somewhat more prewar and postwar context as well as a bit more integration of military theory, Hans van de Ven's China at War: Triumph and Tragedy in the Emergence of the New China (2017) will also be a good introduction, albeit with less specific detail on the war itself.