On this subject I think there are only two choices,
#1 Retreat south to link up with the UK, protect the middle east and hopefully make a last stand at Baghdad/ Cairo.
#2 Retreat East or North East.
IMHO you always choose #2. Make Germany have to split their forces to chase a large Russian stack east and split to offer resistance to a stack of UK units in the south.
IF the USA is in Norway that is even better. USA to the North, large Russian stack that retreated from Moscow in the East and a UK land force in the South. IF Russian retreats south then you just eliminated one threat for Germany. Instead of three fronts it is now only two. Why allow Germany an easier path to victory? Make them choose between 3 options instead of 2.
** Side Note **
If the game gets to this point it is imperative that a Western Invasion happens, does not matter where or in what strength. The Allies need to open up the 4th front on the Western side. Either directly at Germany on continent or going into Italy. Even if the initial invasion is thrown off make sure you have a second invasion ready.
** Foot note **
On the poll question the proper response is not a answer. You retreat East and then North east. Make Germany push units to the top of the board, way out of position. Japan you ask? Do you think Japan has any intentions of pushing 20+ land units to the top of the map and be totally out of position to attack the Russians? It is not just forcing Germany to make a hard choice you are also making Japan make a even harder choice. Japan can ill afford to be putting a large stack in the north of Russia to contend with them, they have larger concerns in this scenario.