From what I can tell you can use either.
The section element used in Webflow is a div with a class “w-section”. It doesn’t seem to do anything other then provide a visual in the designer (could very much be wrong).
When you use either the Section block or the div block you are placing a div tag. When you select section from the html5 options you are converting that div tag to a section tag.
This is how the HTML tags would look;
< div class=“w-section” >(Webflow section block)
< div > (Webflow div block)
< section > (HTML5 markup)
I don’t know if I answered you question, but I hope this helps.
Alex