When white can play e4-e5 it is better not to play c6. The c6 set-up is more common in the 150-attack (Be3, Qd2 - aiming for Bh6 and h4-h5), as it can be used for rapid queenside expansion.
In Pirc Alert (Alburt/Chernin) the (dis)advantages of blacks pieces and pawns is explained in detail in several chapters.
How do you know when to play c6 with black and play for queenside gains with b5 etc, and when to opt for Nc6 and possible central counterplay with e5?