I think there is a curious contrast between The General and The Mule, as storytelling techniques go.
In the first story, the general pits himself against the colossal historical inertia defined and shaped by the Seldon Plan, finding himself in a position that nothing - no matter what he does - can defeat the Foundation. The empire’s internal politics will see to that - if the general is weak, he will be unable to win, while if he is strong his Emperor will turn on him or risk being overthrown himself. That is a pattern we have seen in more contemporary states, such as Saddam’s Iraq, and it stands to reason we would see it here. Asimov also does a good job of subtly demonstrating the Empire’s decline, from the shortage of trained technicians to the backbiting and power struggles in the imperial court. The section in which the two heroes set out to Trantor appears an authorial wrong turning, at least at first, but instead it illustrates how the dead hand of history is positioned against the Empire. There was no need to try to go to Trantor. The Empire was doomed anyway, and individual action means nothing.
In the second, individual action is everything. The Mule has the power to bend everyone to his will, allowing for power combinations that would not seem possible in Seldon’s ordered universe. Imagine Winston Churchill and the British Empire joining Adolf Hitler in 1940 - it would not happen without ASBs, but what is the Mule if not an ASB? The seeming insignificance of the Mule throughout most of the story is matched by the equal insignificance - by 1950s standards - of his nemesis, Bayta. In any of the earlier stories, she would be just another faceless housewife; in this story, where individual action counts, she becomes the heroine who saves the day, laying the groundwork for the Second Foundation to defeat the Mule.
I think Asimov mastered atmosphere, very much so, in the second book. The General is not particularly threatening, partly because we rarely see events through the eyes of the Foundation itself, and you can often find yourself rooting for the General even though he is the bad guy. By contrast, the Mule casts a long shadow over the events of the story, appearing as a near-omnipotent force that drives the heroes from world to world, the mood drastically darkening from the optimism of the early chapters to the fall of Terminus, the Traders, New Trantor and finally Trantor itself. That shadow makes it hard to connect the Mule with his alter ego - and even after the defeat of the resistance on Terminus, which reveals that the Mule himself is not on the planet, it is still hard to make the connection before the denouncement. In hindsight, of course, the signs are there.
Overall, Foundation and Empire is my second favourite of the series.
Chris