Most players consider the game to be quite unbalanced in favor of the Axis. So the two sides will bid (in terms of IPCs) before the game and the lowest bid team will play the Allies. The bid can be used to place additional units before the game starts.
Sea Lion is definitely a much riskier strategy than just going straight for Moscow. Most people only go through with it if London is not well protected after the first round. A failed Sea Lion is pretty much a game ender.