The main difference from a design view is that a PluginOutlet is an insertion point. You inject your component directly into a core template. Every customization actually changes the app structure.
A BlockOutlet is a layout frame. You register one or multiple components with the frame. Then customizations happen within that frame rather than in app templates. So we get more predictable and stable layouts
Another big difference is that the Blocks API can handle the full spectrum of conditional render logic. You don’t need to add that to your component. A block component can focus purely on content and template and we have much less repeated logic.