Difference between Powerful &Ultimate Child Themes

Basically, if you use Divi Ultimate Child Theme on your Divi sites, then you don't have to install those standalone plugins. This is because all the features are built-into the child theme. Those standalone plugins are meant to be used on Divi sites that doesn't use our child theme.

As for the main difference between Divi Ultimate Child Theme & Divi Powerful Child Theme:

• Currently, Divi Ultimate & Divi Powerful have almost the same features. There's still few minor features yet to be added to Divi Ultimate: more sidebar styles, custom 404 page styles, global popup on click, convert last menu item into button & new mobile menu options.
  
• Divi Ultimate has tons of premade layouts / sections. But Divi Powerful doesn't have any premade layouts / sections, except for those premade elements. Divi Powerful is meant to be used together with free ET premade layouts or any Divi layouts out there or built from scratch. Basically, Divi Powerful is sort of a 'bare bones' child theme with tons of features & premade elements to get you stated.