The S.U has less strategic depth (less space to trade for time while mobilizing), less battle experience to get their forces back in shape after the purges.
They may be less of a perceived 'known quantity' to the Germans if they haven't watched them floundering in the Winter War.
It takes a little longer for the Germans to complete the war against Poland. Not sure if the Poles can re-group, hold on for anything more than another few weeks. France had hoped for four months originally I think. Poland has less to whine about being betrayed post-war.
The Baltic countries probably fight in the Axis, get less sympathy post war.