Also the F7c can be equipped with a Gimbaled top turret, while this is not possible on the Ghost.
I've got the Ghost, you can put the F7c-M ball turret on it no problem. It salves to the pilot like the other gimbals, but as there is no second seat for the turret gunner to control it like in the F7c-M that is not an option.
At the moment the biggest difference between the F7c and the F7c-S variant is the colour.
Its supposed to have special armour while less tough is better able to resist scans and detection along with less loud components.
Also, while it has different components in the current alpha build changing components appears to have minimal effect so comparing now is of relatively low benefit.
I recommend spending some REC and renting them, compare them both with your own hands :)