Augustine is more substantial (and saintly) than August, less pretentious than Augustus, and, along with its nickname Gus, is definitely a viable choice.
Augustine is actually the diminutive form of Augustus and achieved its fame via the great fourth century saint, St. Augustine of Hippo, whose teachings and writings had a great effect on early Christianity.
A major character in Uncle Tom's Cabin is Augustine St. Clare. The full name of the author of Ulysses and Finnegans Wake is James Augustine Aloysius Joyce.
Model Linda Evangelista chose the French form Augustin for her son, also used by Jerry Hall and Mick Jagger as a middle name for their son James.
The Spanish Agustin is experiencing a wild burst of popularity in South America: in recent years it has been the number one name in Uruguay and in the Top 5 in Argentina.