I liked how Dooku’s use of force lightning is contrasted with Sidious’. When Dooku uses force lightning against Yoda, he deflects it easily. When Sidious uses it, it sends him flying, and when he is finally able to block it, it takes a lot of strength to do. It’s a great way of showing how Sidious is superior to his (then former) apprentice.
When viewing it in chronological order, Dooku being the first one to be seen using force lightning is to show the audience that he has gained new powers due to joining the dark side. Then in ROTS, we see Sidious has a far more powerful version of the same attack.