Between the two, I would go with Stencil for being more stable, having a good integration with Angular and having very cool features like lazy-loading and differential bundling. Polymer is used on YouTube, which is great, but the migration to LitElement isn't really going fast, showing that it isn't very mature. However, you're using Angular 9. You will probably write your components with it.
What you're probably looking for is actually a component library, which Stencil isn't. What I suggest you take a look instead is Ionic, a web component library built with Stencil, very good alternative to Polymer.