I'd vote go for US now, while Germany is still alive and kicking, assuming they are. If Japan is around too then the US has 3 fronts to deal with, giving an advantage. By spring of 1944, Germany may have fallen.
If i'm wrong and Germany holds out, then it may be best to wait till '44 since the AI will be preparing for D-Day, which means the best American troops will be shipping out, giving you at least a month or two before the enemy returns, perhaps enough time to take plenty of territory.
If i'm wrong and Germany holds out, then it may be best to wait till '44 since the AI will be preparing for D-Day, which means the best American troops will be shipping out, giving you at least a month or two before the enemy returns, perhaps enough time to take plenty of territory.