The question does not really arise. The extension and Reflex do not compete in your day. They respond to different situations. But if you want a simple rule, here it is.
Choose the extension when
- You are on your computer and reading a web page.
- You have chosen this content and want to understand it better.
- You have time to read, but want a boost on a difficult passage.
- You are working in your browser and want to stay in your workflow.
Choose Reflex when
- You are on your phone and a text has just arrived.
- You do not yet know whether this text deserves your attention.
- You do not have access to your computer or you are on the move.
- You want to process a document, ebook or file imported from your phone.
- You want to listen to a result aloud rather than read it.
In practice
Most users who have both no longer ask the question. The context chooses for them. In front of a computer screen with a browser open, it is the extension. Phone in hand with a text that has just arrived, it is Reflex.
If you only have one of the two, the answer is even simpler. The extension covers your online reading. Reflex covers everything else.