From: Madrid, Spain
Well, neither is very easy to learn... maybe AACW is a little easier than FOF.
But the main diffrerence between them is the focus.
FOF is grand strategic (politics, production, economy, big regions) with tactical battles (fought on an hex map by turns).
AACW is more operational. You have less politics/porduction possibilities and don't have playable tactical battles but excels in the in bettween scale, military operations: many more regions, and detail on the army structure, rivers, roads, rails, and terrain have much more importance on how you plan and fight your campaigns.
Personaly, i prefer AACW