Hi Walter 👋
It's a pretty common question and the answer is a bit nuanced, but at a high level glamorous uses objects for authoring the CSS and styled-components uses tagged template literals (strings) for authoring the CSS.
You can read about why I created glamorous in the intro blog post. Basically it came down to performance, size, preference, and the ability to apply RTL conversion on the CSS. Also, with styled-components you have to give a name to every element you want to style (by creating a component) whereas with glamorous there are the built-in components which allow you to add the styles directly to the elements and not have to give them a name (like Wrapper or Container). I also think that the glamorous APIs compose a little better than styled-components.
Good luck to you!