It looks wonderful! But I am not sure if I can answer your question since I haven't read it. =) Would seeing these side by side help??
Here is what YC says about The Story of Rome:
The Story of Rome by Mary Macgregor A vivid account of the story of Rome from the earliest times to the death of Augustus, retold for children, chronicling the birth of a city and its growth through storm and struggle to become a great world empire. Gives short accounts of battles and campaigns, and of the men who expanded the borders of the Roman empire to include all lands bordering the Mediterranean Sea. Ages 10-14 593 pages
The Story of the Romans by Helene A. Guerber Elementary history of Rome, presenting short stories of the great heroes, mythical and historical, from Aeneas and the founding of Rome to the fall of the western empire. Around the famous characters of Rome are graphically grouped the great events with which their names will forever stand connected. Vivid descriptions bring to life the events narrated, making history attractive to the young, and awakening their enthusiasm for further reading and study. Ages 10-14349 pages
Here's the blurb about ACW from Beautiful Feet -
Augustus Caesar's World - by Genevieve Foster - Published by Beautiful Feet Books
Considered the innovator of "horizontal history," Genevieve Foster became frustrated when her two school-aged children complained about the boring presentation of history in their school texts. This frustration led to Foster's first book, George Washington's World (1941). In her unique approach, Foster weaves a story of the world around her central character; rather than focusing exclusively on geo-political events, as most textbooks do; she includes stories of scientific discovery and invention, music, literature, art, and religion. She has a keen intuition for stories that will especially delight and amuse her youthful audience. InAugustus Caesar's World, Foster traces the seven major civilizations—Rome, Greece, Israel, Egypt, China, India, and Persia—from 4500 B.C. to the time of Augustus Caesar in 44 B.C. and culminating in 14 A.D. Within this timeframe readers will learn not only the stories of Julius Caesar, Cleopatra, and Marc Antony, but also the historian Livy and how Virgil came to write the Aeneid. Foster will then take her readers all over the world to learn what was happening at this same time in China, Persia, India and so on. Foster's detailed pen and ink drawings are fresh and appealing, and her illustrated timelines give a clear sense of chronology, enriching the engaging text. An all-time customer favorite! It has 340 pages.
Think about your son, his enjoyment of reading (perhaps his level?) and then consider the time frame you are allowing per civilization. Will that help??
Are your younger sons going to be studying them as well?