Built-in VS Imported Animations

Hi everyone,
I'm wondering if there's one way of making animations in unity that one should preferably use between Built-In and Imported animations. Particularly, I'm wondering if there's a significant difference in performances between the two. My project is still is in an early stage but I want to plan things out. I'm 95% sure that no scene is likely to have more than 10 animated characters at once.