Not familiar with that tool. What follows is my best guess based on what I am seeing.

Serena looks to be a precision tool. Since it uses uses LSP its able to replicate a lot of what a IDE would allow and IDE for LLM's.

cs by contrast is more of a discovery tool. When you're trying to find where the work actually happens it can help you, and since there is no index involved you can get going instantly on any codebase while they are index.

You could use cs for instant to find where the complexity lies, and then use Serena to modify it.