In classic mode you can select less nations, there is no media and a lot of things are optional (like for example training). This results in a much faster and trimmed down experience. However if you want the full in depth experience with a lot of active nations and that touches on everything a manager does in real life (where the media plays an important role, you need to talk to your players, you need to study your opponent) then full mode is the one you want. One could argue that the focus in classic mode lies on tactics and transfers