I think who would have won would depend on who attacked.
If Germany had attacked it would haven been almost like it was in the war except that if we don't include the allies sending them supplies then the germans might have won the war. But if the soviets had managed to move everything far to the east(which they actually did, except the regime ofcourse) they would still have had a chance against the germans.
If the Soviets had invaded Germany it would have been a different matter, that I am sure of, first the soviets would be better prepared to fight, plainly because they had then been ordered to fight, lots of russians didn't dare offer the germans any resistance because they had not yet got the order from Stalin to do so, and Stalin did not like people taking initiative without his approval. The soviets would still have suffered lots of casualties due to their many incompetent commanders who had no real experience or knowledge of commanding(many had just been NCOs and not officers), and ofcourse their poorly trained, and often underequipped, soldiers(which was because Stalin had killed all the men responsible for the training of the russian soldiers).
In this scenario I am pretty certain the Soviets would have won the war.

As to who I would support, I think I would root for the Soviets because, to quote Churchill "If Hitler had invaded hell I would have sided with the devil"