both things are totally different. The page editor helps you to fill posts/pages and portfolio entries with content (images, text, etc.). You can use shortcodes, etc. too.
The template builder generates custom page layouts based on the elements you’ve chosen for the particular template. I.e. you can display the content of several pages on one specific page (by using the “Post/Page content” element but you can also combine portfolio elements with slideshows and static text, etc.
So the difference is that the template builder can generate custom layouts (and the content depends on the elements you use) and the editor just inserts text into the content area of a page or post.