Communism ftw. Always now, especially if you can get the great spy and Kremlin.
This allows you to make a super espionage city. Put Kremlin, scotland yard, courthouse, intelligence agency, bureau, and jail in a high-food city (you may need to rush-buy Kremlin for around $6000) and you will be able to run 9 spy specialists pumping out in the neighbourhood of 200-300 ep/turn. This can manage your entire espionage efforts throughout the game.
Physics is good for getting the free scientist, but given a choice I would rather get my super-espionage city set up.