Keegan, who died recently, was a leading military historian, and his book is a superb survey of the war as military history. In contrast, Ferguson, by training an academic economist, devotes his book to an analysis of the economic and strategic factors that drove the war.
Notably, both books offered some revisionist conclusions. Keegan mostly rehabilitated General Douglas Haig's extraordinarily costly attacks on the Somme, while Ferguson treats British involvement in the war as a grave mistake that directly led to the long stalemate, massive loss of life, and political disruptions that the war set in motion.
Thanks for the recommendations. Will be happy to add those to my list.